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/renderer.go

@@ -30,14 +30,16 @@ const (
 
 type ProcessorHelper struct {
 	IsUsernameMentionable func(ctx context.Context, username string) bool
+
+	ElementDir string // the direction of the elements, eg: "ltr", "rtl", "auto", default to no direction attribute
 }
 
-var processorHelper ProcessorHelper
+var DefaultProcessorHelper ProcessorHelper
 
 // Init initialize regexps for markdown parsing
 func Init(ph *ProcessorHelper) {
 	if ph != nil {
-		processorHelper = *ph
+		DefaultProcessorHelper = *ph
 	}
 
 	NewSanitizer()