Add KaTeX rendering to Markdown. (#20571)

This PR adds mathematical rendering with KaTeX.

The first step is to add a Goldmark extension that detects the latex
(and tex) mathematics delimiters.

The second step to make this extension only run if math support is
enabled.

The second step is to then add KaTeX CSS and JS to the head which will
load after the dom is rendered.

Fix #3445

Signed-off-by: Andrew Thornton <art27@cantab.net>

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/markup/markdown/math/block_renderer.go

@@ -0,0 +1,43 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package math
+
+import (
+	gast "github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/renderer"
+	"github.com/yuin/goldmark/util"
+)
+
+// BlockRenderer represents a renderer for math Blocks
+type BlockRenderer struct{}
+
+// NewBlockRenderer creates a new renderer for math Blocks
+func NewBlockRenderer() renderer.NodeRenderer {
+	return &BlockRenderer{}
+}
+
+// RegisterFuncs registers the renderer for math Blocks
+func (r *BlockRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
+	reg.Register(KindBlock, r.renderBlock)
+}
+
+func (r *BlockRenderer) writeLines(w util.BufWriter, source []byte, n gast.Node) {
+	l := n.Lines().Len()
+	for i := 0; i < l; i++ {
+		line := n.Lines().At(i)
+		_, _ = w.Write(util.EscapeHTML(line.Value(source)))
+	}
+}
+
+func (r *BlockRenderer) renderBlock(w util.BufWriter, source []byte, node gast.Node, entering bool) (gast.WalkStatus, error) {
+	n := node.(*Block)
+	if entering {
+		_, _ = w.WriteString(`<pre class="code-block is-loading"><code class="chroma language-math display">`)
+		r.writeLines(w, source, n)
+	} else {
+		_, _ = w.WriteString(`</code></pre>` + "\n")
+	}
+	return gast.WalkContinue, nil
+}