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_parser.go

@@ -0,0 +1,99 @@
+// 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 (
+	"bytes"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/parser"
+	"github.com/yuin/goldmark/text"
+)
+
+type inlineParser struct {
+	start []byte
+	end   []byte
+}
+
+var defaultInlineDollarParser = &inlineParser{
+	start: []byte{'$'},
+	end:   []byte{'$'},
+}
+
+// NewInlineDollarParser returns a new inline parser
+func NewInlineDollarParser() parser.InlineParser {
+	return defaultInlineDollarParser
+}
+
+var defaultInlineBracketParser = &inlineParser{
+	start: []byte{'\\', '('},
+	end:   []byte{'\\', ')'},
+}
+
+// NewInlineDollarParser returns a new inline parser
+func NewInlineBracketParser() parser.InlineParser {
+	return defaultInlineBracketParser
+}
+
+// Trigger triggers this parser on $
+func (parser *inlineParser) Trigger() []byte {
+	return parser.start[0:1]
+}
+
+func isAlphanumeric(b byte) bool {
+	// Github only cares about 0-9A-Za-z
+	return (b >= '0' && b <= '9') || (b >= 'A' && b <= 'Z') || (b >= 'a' && b <= 'z')
+}
+
+// Parse parses the current line and returns a result of parsing.
+func (parser *inlineParser) Parse(parent ast.Node, block text.Reader, pc parser.Context) ast.Node {
+	line, _ := block.PeekLine()
+	opener := bytes.Index(line, parser.start)
+	if opener < 0 {
+		return nil
+	}
+	if opener != 0 && isAlphanumeric(line[opener-1]) {
+		return nil
+	}
+
+	opener += len(parser.start)
+	ender := bytes.Index(line[opener:], parser.end)
+	if ender < 0 {
+		return nil
+	}
+	if len(line) > opener+ender+len(parser.end) && isAlphanumeric(line[opener+ender+len(parser.end)]) {
+		return nil
+	}
+
+	block.Advance(opener)
+	_, pos := block.Position()
+	node := NewInline()
+	segment := pos.WithStop(pos.Start + ender)
+	node.AppendChild(node, ast.NewRawTextSegment(segment))
+	block.Advance(ender + len(parser.end))
+
+	trimBlock(node, block)
+	return node
+}
+
+func trimBlock(node *Inline, block text.Reader) {
+	if node.IsBlank(block.Source()) {
+		return
+	}
+
+	// trim first space and last space
+	first := node.FirstChild().(*ast.Text)
+	if !(!first.Segment.IsEmpty() && block.Source()[first.Segment.Start] == ' ') {
+		return
+	}
+
+	last := node.LastChild().(*ast.Text)
+	if !(!last.Segment.IsEmpty() && block.Source()[last.Segment.Stop-1] == ' ') {
+		return
+	}
+
+	first.Segment = first.Segment.WithStart(first.Segment.Start + 1)
+	last.Segment = last.Segment.WithStop(last.Segment.Stop - 1)
+}