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/inline_node.go

@@ -0,0 +1,49 @@
+// 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 (
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/util"
+)
+
+// Inline represents inline math e.g. $...$ or \(...\)
+type Inline struct {
+	ast.BaseInline
+}
+
+// Inline implements Inline.Inline.
+func (n *Inline) Inline() {}
+
+// IsBlank returns if this inline node is empty
+func (n *Inline) IsBlank(source []byte) bool {
+	for c := n.FirstChild(); c != nil; c = c.NextSibling() {
+		text := c.(*ast.Text).Segment
+		if !util.IsBlank(text.Value(source)) {
+			return false
+		}
+	}
+	return true
+}
+
+// Dump renders this inline math as debug
+func (n *Inline) Dump(source []byte, level int) {
+	ast.DumpHelper(n, source, level, nil, nil)
+}
+
+// KindInline is the kind for math inline
+var KindInline = ast.NewNodeKind("MathInline")
+
+// Kind returns KindInline
+func (n *Inline) Kind() ast.NodeKind {
+	return KindInline
+}
+
+// NewInline creates a new ast math inline node
+func NewInline() *Inline {
+	return &Inline{
+		BaseInline: ast.BaseInline{},
+	}
+}