tocss: Add vars option

This commit adds a new `vars` option to both the Sass transpilers (Dart Sass and Libsass).

This means that you can pass a map with key/value pairs to the transpiler:

```handlebars
{{ $vars := dict "$color1" "blue" "$color2" "green" "$font_size" "24px" }}
{{ $cssOpts := (dict "transpiler" "dartsass" "outputStyle" "compressed" "vars" $vars ) }}
{{ $r := resources.Get "scss/main.scss" |  toCSS $cssOpts }}
```

And the the variables will be available in the `hugo:vars` namespace. Example usage for Dart Sass:

```scss
@use "hugo:vars" as v;

p {
    color: v.$color1;
    font-size: v.$font_size;
}
```

Note that Libsass does not support the `use` keyword, so you need to `import` them as global variables:

```scss
@import "hugo:vars";

p {
    color: $color1;
    font-size: $font_size;
}
```

Hugo will:

* Add a missing leading `$` for the variable names if needed.
* Wrap the values in `unquote('VALUE')` (Sass built-in) to get proper handling of identifiers vs other strings.

This means that you can pull variables directly from e.g. the site config:

```toml
[params]
[params.sassvars]
color1 = "blue"
color2 = "green"
font_size = "24px"
image = "images/hero.jpg"
```

```handlebars
{{ $vars := site.Params.sassvars}}
{{ $cssOpts := (dict "transpiler" "dartsass" "outputStyle" "compressed" "vars" $vars ) }}
{{ $r := resources.Get "scss/main.scss" |  toCSS $cssOpts }}
```

Fixes #10555

resources/resource_transformers/tocss/dartsass/client.go

@@ -93,6 +93,7 @@ func (c *Client) toCSS(args godartsass.Args, src io.Reader) (godartsass.Result,
 	var res godartsass.Result
 
 	in := helpers.ReaderToString(src)
+
 	args.Source = in
 
 	res, err := c.transpiler.Execute(args)
@@ -130,6 +131,11 @@ type Options struct {
 
 	// If enabled, sources will be embedded in the generated source map.
 	SourceMapIncludeSources bool
+
+	// Vars will be available in 'hugo:vars', e.g:
+	//     @use "hugo:vars";
+	//     $color: vars.$color;
+	Vars map[string]string
 }
 
 func decodeOptions(m map[string]any) (opts Options, err error) {

resources/resource_transformers/tocss/dartsass/integration_test.go

@@ -24,6 +24,7 @@ import (
 )
 
 func TestTransformIncludePaths(t *testing.T) {
+	t.Parallel()
 	if !dartsass.Supports() {
 		t.Skip()
 	}
@@ -55,6 +56,7 @@ T1: {{ $r.Content }}
 }
 
 func TestTransformImportRegularCSS(t *testing.T) {
+	t.Parallel()
 	if !dartsass.Supports() {
 		t.Skip()
 	}
@@ -108,6 +110,7 @@ T1: {{ $r.Content | safeHTML }}
 }
 
 func TestTransformThemeOverrides(t *testing.T) {
+	t.Parallel()
 	if !dartsass.Supports() {
 		t.Skip()
 	}
@@ -169,6 +172,7 @@ zoo {
 }
 
 func TestTransformLogging(t *testing.T) {
+	t.Parallel()
 	if !dartsass.Supports() {
 		t.Skip()
 	}
@@ -200,6 +204,7 @@ T1: {{ $r.Content }}
 }
 
 func TestTransformErrors(t *testing.T) {
+	t.Parallel()
 	if !dartsass.Supports() {
 		t.Skip()
 	}
@@ -271,3 +276,93 @@ T1: {{ $r.Content }}
 	})
 
 }
+
+func TestOptionVars(t *testing.T) {
+	t.Parallel()
+	if !dartsass.Supports() {
+		t.Skip()
+	}
+
+	files := `
+-- assets/scss/main.scss --
+@use "hugo:vars";
+
+body {
+	body {
+		background: url(vars.$image) no-repeat center/cover;
+	  }	  
+}
+
+p {
+	color: vars.$color1;
+	font-size: vars.$font_size;
+}
+
+b {
+	color: vars.$color2;
+}
+-- layouts/index.html --
+{{ $image := "images/hero.jpg" }}
+{{ $vars := dict "$color1" "blue" "$color2" "green" "font_size" "24px" "image" $image }}
+{{ $cssOpts := (dict "transpiler" "dartsass" "outputStyle" "compressed" "vars" $vars ) }}
+{{ $r := resources.Get "scss/main.scss" |  toCSS $cssOpts }}
+T1: {{ $r.Content }}
+	`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			NeedsOsFS:   true,
+		}).Build()
+
+	b.AssertFileContent("public/index.html", `T1: body body{background:url(images/hero.jpg) no-repeat center/cover}p{color:blue;font-size:24px}b{color:green}`)
+}
+
+func TestOptionVarsParams(t *testing.T) {
+	t.Parallel()
+	if !dartsass.Supports() {
+		t.Skip()
+	}
+
+	files := `
+-- config.toml --
+[params]
+[params.sassvars]
+color1 = "blue"
+color2 = "green"
+font_size = "24px"
+image = "images/hero.jpg"
+-- assets/scss/main.scss --
+@use "hugo:vars";
+
+body {
+	body {
+		background: url(vars.$image) no-repeat center/cover;
+	  }	  
+}
+
+p {
+	color: vars.$color1;
+	font-size: vars.$font_size;
+}
+
+b {
+	color: vars.$color2;
+}
+-- layouts/index.html --
+{{ $vars := site.Params.sassvars}}
+{{ $cssOpts := (dict "transpiler" "dartsass" "outputStyle" "compressed" "vars" $vars ) }}
+{{ $r := resources.Get "scss/main.scss" |  toCSS $cssOpts }}
+T1: {{ $r.Content }}
+	`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			NeedsOsFS:   true,
+		}).Build()
+
+	b.AssertFileContent("public/index.html", `T1: body body{background:url(images/hero.jpg) no-repeat center/cover}p{color:blue;font-size:24px}b{color:green}`)
+}

resources/resource_transformers/tocss/dartsass/transform.go

@@ -1,4 +1,4 @@
-// Copyright 2020 The Hugo Authors. All rights reserved.
+// Copyright 2022 The Hugo Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -28,6 +28,7 @@ import (
 	"github.com/gohugoio/hugo/resources"
 
 	"github.com/gohugoio/hugo/resources/internal"
+	"github.com/gohugoio/hugo/resources/resource_transformers/tocss/internal/sass"
 
 	"github.com/spf13/afero"
 
@@ -84,6 +85,8 @@ func (t *transform) Transform(ctx *resources.ResourceTransformationCtx) error {
 		ImportResolver: importResolver{
 			baseDir: baseDir,
 			c:       t.c,
+
+			varsStylesheet: sass.CreateVarsStyleSheet(opts.Vars),
 		},
 		OutputStyle:             godartsass.ParseOutputStyle(opts.OutputStyle),
 		EnableSourceMap:         opts.EnableSourceMap,
@@ -128,9 +131,14 @@ func (t *transform) Transform(ctx *resources.ResourceTransformationCtx) error {
 type importResolver struct {
 	baseDir string
 	c       *Client
+
+	varsStylesheet string
 }
 
 func (t importResolver) CanonicalizeURL(url string) (string, error) {
+	if url == sass.HugoVarsNamespace {
+		return url, nil
+	}
 	filePath, isURL := paths.UrlToFilename(url)
 	var prevDir string
 	var pathDir string
@@ -177,6 +185,9 @@ func (t importResolver) CanonicalizeURL(url string) (string, error) {
 }
 
 func (t importResolver) Load(url string) (string, error) {
+	if url == sass.HugoVarsNamespace {
+		return t.varsStylesheet, nil
+	}
 	filename, _ := paths.UrlToFilename(url)
 	b, err := afero.ReadFile(hugofs.Os, filename)
 	return string(b), err