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_link.go

@@ -0,0 +1,47 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markdown
+
+import (
+	"bytes"
+	"slices"
+
+	"code.gitea.io/gitea/modules/markup"
+	"code.gitea.io/gitea/modules/setting"
+	giteautil "code.gitea.io/gitea/modules/util"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
+)
+
+func (g *ASTTransformer) transformLink(ctx *markup.RenderContext, v *ast.Link, reader text.Reader) {
+	// Links need their href to munged to be a real value
+	link := v.Destination
+
+	// Do not process the link if it's not a link, starts with an hashtag
+	// (indicating it's an anchor link), starts with `mailto:` or any of the
+	// custom markdown URLs.
+	processLink := len(link) > 0 && !markup.IsLink(link) &&
+		link[0] != '#' && !bytes.HasPrefix(link, byteMailto) &&
+		!slices.ContainsFunc(setting.Markdown.CustomURLSchemes, func(s string) bool {
+			return bytes.HasPrefix(link, []byte(s+":"))
+		})
+
+	if processLink {
+		var base string
+		if ctx.IsWiki {
+			base = ctx.Links.WikiLink()
+		} else if ctx.Links.HasBranchInfo() {
+			base = ctx.Links.SrcLink()
+		} else {
+			base = ctx.Links.Base
+		}
+
+		link = []byte(giteautil.URLJoin(base, string(link)))
+	}
+	if len(link) > 0 && link[0] == '#' {
+		link = []byte("#user-content-" + string(link)[1:])
+	}
+	v.Destination = link
+}