Add RTL rendering support to Markdown (#24816)

Support RTL content in Markdown:


![image](https://github.com/go-gitea/gitea/assets/115237/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>

services/markup/processorhelper.go

@@ -13,6 +13,7 @@ import (
 
 func ProcessorHelper() *markup.ProcessorHelper {
 	return &markup.ProcessorHelper{
+		ElementDir: "auto", // set dir="auto" for necessary (eg: <p>, <h?>, etc) tags
 		IsUsernameMentionable: func(ctx context.Context, username string) bool {
 			mentionedUser, err := user.GetUserByName(ctx, username)
 			if err != nil {