Add Markdown diagrams and render hooks for code blocks

You can now create custom hook templates for code blocks, either one for all (`render-codeblock.html`) or for a given code language (e.g. `render-codeblock-go.html`).

We also used this new hook to add support for diagrams in Hugo:

* Goat (Go ASCII Tool) is built-in and enabled by default; just create a fenced code block with the language `goat` and start draw your Ascii diagrams.
* Another popular alternative for diagrams in Markdown, Mermaid (supported by GitHub), can also be implemented with a simple template. See the Hugo documentation for more information.

Updates #7765
Closes #9538
Fixes #9553
Fixes #8520
Fixes #6702
Fixes #9558

tpl/diagrams/diagrams.go

@@ -0,0 +1,73 @@
+// 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.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package diagrams
+
+import (
+	"bytes"
+	"html/template"
+	"io"
+	"strings"
+
+	"github.com/bep/goat"
+	"github.com/gohugoio/hugo/deps"
+	"github.com/spf13/cast"
+)
+
+type SVGDiagram interface {
+	Body() template.HTML
+	SVG() template.HTML
+	Width() int
+	Height() int
+}
+
+type goatDiagram struct {
+	d goat.SVG
+}
+
+func (d goatDiagram) Body() template.HTML {
+	return template.HTML(d.d.Body)
+}
+
+func (d goatDiagram) SVG() template.HTML {
+	return template.HTML(d.d.String())
+}
+
+func (d goatDiagram) Width() int {
+	return d.d.Width
+}
+
+func (d goatDiagram) Height() int {
+	return d.d.Height
+}
+
+type Diagrams struct {
+	d *deps.Deps
+}
+
+func (d *Diagrams) Goat(v interface{}) SVGDiagram {
+	var r io.Reader
+
+	switch vv := v.(type) {
+	case io.Reader:
+		r = vv
+	case []byte:
+		r = bytes.NewReader(vv)
+	default:
+		r = strings.NewReader(cast.ToString(v))
+	}
+
+	return goatDiagram{
+		d: goat.BuildSVG(r),
+	}
+}