Add RTL rendering support to Markdown (#24816)

Support RTL content in Markdown:


![image](dedb1b0c-2f05-40dc-931a-0d9dc81f7c97)

Example document:
https://try.gitea.io/silverwind/symlink-test/src/branch/master/bidi-text.md
Same on GitHub:
https://github.com/silverwind/symlink-test/blob/master/bidi-text.md

`dir=auto` enables a browser heuristic that sets the text direction
automatically. It is the only way to get automatic text direction.

Ref: https://codeberg.org/Codeberg/Community/issues/1021

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/markdown/goldmark.go

@@ -47,6 +47,12 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 		tocMode = rc.TOC
 	}
 
+	applyElementDir := func(n ast.Node) {
+		if markup.DefaultProcessorHelper.ElementDir != "" {
+			n.SetAttributeString("dir", []byte(markup.DefaultProcessorHelper.ElementDir))
+		}
+	}
+
 	attentionMarkedBlockquotes := make(container.Set[*ast.Blockquote])
 	_ = ast.Walk(node, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
 		if !entering {
@@ -69,6 +75,9 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 				header.ID = util.BytesToReadOnlyString(id.([]byte))
 			}
 			tocList = append(tocList, header)
+			applyElementDir(v)
+		case *ast.Paragraph:
+			applyElementDir(v)
 		case *ast.Image:
 			// Images need two things:
 			//
@@ -171,6 +180,7 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 					v.AppendChild(v, newChild)
 				}
 			}
+			applyElementDir(v)
 		case *ast.Text:
 			if v.SoftLineBreak() && !v.HardLineBreak() {
 				renderMetas := pc.Get(renderMetasKey).(map[string]string)