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

markup/goldmark/codeblocks/integration_test.go

@@ -0,0 +1,115 @@
+// 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 codeblocks_test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/gohugoio/hugo/hugolib"
+)
+
+func TestCodeblocks(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+[markup]
+  [markup.highlight]
+    anchorLineNos = false
+    codeFences = true
+    guessSyntax = false
+    hl_Lines = ''
+    lineAnchors = ''
+    lineNoStart = 1
+    lineNos = false
+    lineNumbersInTable = true
+    noClasses = false
+    style = 'monokai'
+    tabWidth = 4
+-- layouts/_default/_markup/render-codeblock-goat.html --
+{{ $diagram := diagrams.Goat .Code }}
+Goat SVG:{{ substr $diagram.SVG 0 100 | safeHTML }}  }}|
+Goat Attribute: {{ .Attributes.width}}|
+-- layouts/_default/_markup/render-codeblock-go.html --
+Go Code: {{ .Code | safeHTML }}|
+Go Language: {{ .Lang }}|
+-- layouts/_default/single.html --
+{{ .Content }}
+-- content/p1.md --
+---
+title: "p1"
+---
+
+## Ascii Diagram
+
+CODE_FENCEgoat { width="600" }
+--->
+CODE_FENCE
+
+## Go Code
+
+CODE_FENCEgo
+fmt.Println("Hello, World!");
+CODE_FENCE
+
+## Golang Code
+
+CODE_FENCEgolang
+fmt.Println("Hello, Golang!");
+CODE_FENCE
+
+## Bash Code
+
+CODE_FENCEbash { linenos=inline,hl_lines=[2,"5-6"],linenostart=32 class=blue }
+echo "l1";
+echo "l2";
+echo "l3";
+echo "l4";
+echo "l5";
+echo "l6";
+echo "l7";
+echo "l8";
+CODE_FENCE
+`
+
+	files = strings.ReplaceAll(files, "CODE_FENCE", "```")
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			NeedsOsFS:   false,
+		},
+	).Build()
+
+	b.AssertFileContent("public/p1/index.html", `
+Goat SVG:<svg class='diagram'
+Goat Attribute: 600|
+
+Go Language: go|
+Go Code: fmt.Println("Hello, World!");
+
+Go Code: fmt.Println("Hello, Golang!");
+Go Language: golang|
+
+
+	`,
+		"Goat SVG:<svg class='diagram' xmlns='http://www.w3.org/2000/svg' version='1.1' height='25' width='40'",
+		"Goat Attribute: 600|",
+		"<h2 id=\"go-code\">Go Code</h2>\nGo Code: fmt.Println(\"Hello, World!\");\n|\nGo Language: go|",
+		"<h2 id=\"golang-code\">Golang Code</h2>\nGo Code: fmt.Println(\"Hello, Golang!\");\n|\nGo Language: golang|",
+		"<h2 id=\"bash-code\">Bash Code</h2>\n<div class=\"highlight blue\"><pre tabindex=\"0\" class=\"chroma\"><code class=\"language-bash\" data-lang=\"bash\"><span class=\"line\"><span class=\"ln\">32</span><span class=\"cl\"><span class=\"nb\">echo</span> <span class=\"s2\">&#34;l1&#34;</span><span class=\"p\">;</span>\n</span></span><span class=\"line hl\"><span class=\"ln\">33</span>",
+	)
+}