Refactor markup render system (#32612)

This PR removes (almost) all path tricks, and introduces "renderhelper"
package.

Now we can clearly see the rendering behaviors for comment/file/wiki,
more details are in "renderhelper" tests.

Fix #31411 , fix #18592, fix #25632 and maybe more problems. (ps: fix
#32608 by the way)

modules/markup/renderer.go

@@ -6,7 +6,7 @@ package markup
 import (
 	"bytes"
 	"io"
-	"path/filepath"
+	"path"
 	"strings"
 
 	"code.gitea.io/gitea/modules/setting"
@@ -55,7 +55,7 @@ func RegisterRenderer(renderer Renderer) {
 
 // GetRendererByFileName get renderer by filename
 func GetRendererByFileName(filename string) Renderer {
-	extension := strings.ToLower(filepath.Ext(filename))
+	extension := strings.ToLower(path.Ext(filename))
 	return extRenderers[extension]
 }