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_codespan.go

@@ -0,0 +1,56 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markdown
+
+import (
+	"bytes"
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/modules/markup"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/renderer/html"
+	"github.com/yuin/goldmark/text"
+	"github.com/yuin/goldmark/util"
+)
+
+// renderCodeSpan renders CodeSpan elements (like goldmark upstream does) but also renders ColorPreview elements.
+// See #21474 for reference
+func (r *HTMLRenderer) renderCodeSpan(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		if n.Attributes() != nil {
+			_, _ = w.WriteString("<code")
+			html.RenderAttributes(w, n, html.CodeAttributeFilter)
+			_ = w.WriteByte('>')
+		} else {
+			_, _ = w.WriteString("<code>")
+		}
+		for c := n.FirstChild(); c != nil; c = c.NextSibling() {
+			switch v := c.(type) {
+			case *ast.Text:
+				segment := v.Segment
+				value := segment.Value(source)
+				if bytes.HasSuffix(value, []byte("\n")) {
+					r.Writer.RawWrite(w, value[:len(value)-1])
+					r.Writer.RawWrite(w, []byte(" "))
+				} else {
+					r.Writer.RawWrite(w, value)
+				}
+			case *ColorPreview:
+				_, _ = w.WriteString(fmt.Sprintf(`<span class="color-preview" style="background-color: %v"></span>`, string(v.Color)))
+			}
+		}
+		return ast.WalkSkipChildren, nil
+	}
+	_, _ = w.WriteString("</code>")
+	return ast.WalkContinue, nil
+}
+
+func (g *ASTTransformer) transformCodeSpan(ctx *markup.RenderContext, v *ast.CodeSpan, reader text.Reader) {
+	colorContent := v.Text(reader.Source())
+	if matchColor(strings.ToLower(string(colorContent))) {
+		v.AppendChild(v, NewColorPreview(colorContent))
+	}
+}