Refactor markdown render (#30139)

Only split the file into small ones (and rename AttentionTypes to
attentionTypes)

(cherry picked from commit 71706126b56616750a65290460fd211b9b8449da)
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>

Conflicts:
	- modules/markup/markdown/goldmark.go
	- modules/markup/markdown/transform_blockquote.go
	Conflicts were resolved by favouring the Forgejo implementation:
	I copied the Forgejo code to the same place Gitea copied them
	to, and adjusted the imports accordingly.

	Apart from conflict resolution, this also moves
	`applyElementDir` from a local func in `goldmark.Transform` to a
	method on `*ASTTransformer`, to make it callable from the
	extracted functions.

modules/markup/markdown/transform_heading.go

@@ -0,0 +1,32 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markdown
+
+import (
+	"fmt"
+
+	"code.gitea.io/gitea/modules/markup"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+func (g *ASTTransformer) transformHeading(ctx *markup.RenderContext, v *ast.Heading, reader text.Reader, tocList *[]markup.Header) {
+	for _, attr := range v.Attributes() {
+		if _, ok := attr.Value.([]byte); !ok {
+			v.SetAttribute(attr.Name, []byte(fmt.Sprintf("%v", attr.Value)))
+		}
+	}
+	txt := v.Text(reader.Source())
+	header := markup.Header{
+		Text:  util.BytesToReadOnlyString(txt),
+		Level: v.Level,
+	}
+	if id, found := v.AttributeString("id"); found {
+		header.ID = util.BytesToReadOnlyString(id.([]byte))
+	}
+	*tocList = append(*tocList, header)
+	g.applyElementDir(v)
+}