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_image.go

@@ -0,0 +1,66 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markdown
+
+import (
+	"strings"
+
+	"code.gitea.io/gitea/modules/markup"
+	giteautil "code.gitea.io/gitea/modules/util"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/text"
+)
+
+func (g *ASTTransformer) transformImage(ctx *markup.RenderContext, v *ast.Image, reader text.Reader) {
+	// Images need two things:
+	//
+	// 1. Their src needs to munged to be a real value
+	// 2. If they're not wrapped with a link they need a link wrapper
+
+	// Check if the destination is a real link
+	if len(v.Destination) > 0 && !markup.IsLink(v.Destination) {
+		v.Destination = []byte(giteautil.URLJoin(
+			ctx.Links.ResolveMediaLink(ctx.IsWiki),
+			strings.TrimLeft(string(v.Destination), "/"),
+		))
+	}
+
+	parent := v.Parent()
+	// Create a link around image only if parent is not already a link
+	if _, ok := parent.(*ast.Link); !ok && parent != nil {
+		next := v.NextSibling()
+
+		// Create a link wrapper
+		wrap := ast.NewLink()
+		wrap.Destination = v.Destination
+		wrap.Title = v.Title
+		wrap.SetAttributeString("target", []byte("_blank"))
+
+		// Duplicate the current image node
+		image := ast.NewImage(ast.NewLink())
+		image.Destination = v.Destination
+		image.Title = v.Title
+		for _, attr := range v.Attributes() {
+			image.SetAttribute(attr.Name, attr.Value)
+		}
+		for child := v.FirstChild(); child != nil; {
+			next := child.NextSibling()
+			image.AppendChild(image, child)
+			child = next
+		}
+
+		// Append our duplicate image to the wrapper link
+		wrap.AppendChild(wrap, image)
+
+		// Wire in the next sibling
+		wrap.SetNextSibling(next)
+
+		// Replace the current node with the wrapper link
+		parent.ReplaceChild(parent, v, wrap)
+
+		// But most importantly ensure the next sibling is still on the old image too
+		v.SetNextSibling(next)
+	}
+}