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) {