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/sanitizer_default_test.go

@@ -19,6 +19,7 @@ func TestSanitizer(t *testing.T) {
 		// Code highlighting class
 		`<code class="random string"></code>`, `<code></code>`,
 		`<code class="language-random ui tab active menu attached animating sidebar following bar center"></code>`, `<code></code>`,
+		`<span class="k"></span><span class="nb"></span>`, `<span class="k"></span><span class="nb"></span>`,
 
 		// Input checkbox
 		`<input type="hidden">`, ``,