Fix inconsistent rendering of block mathematical expressions (#29677)

Fix #28735

GitHub render `\```math\``` ` as a block now.
Add `display` class will render it as a block.

After:

![image](https://github.com/go-gitea/gitea/assets/18380374/2a1c20c7-438e-4ab1-8c66-cf91c8343087)

![image](https://github.com/go-gitea/gitea/assets/18380374/b81b8a93-8bca-46a5-b7db-e0d2f53e1342)

---------

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

modules/markup/markdown/markdown.go

@@ -104,7 +104,8 @@ func SpecializedMarkdown() goldmark.Markdown {
 							}
 
 							// include language-x class as part of commonmark spec
-							_, err = w.WriteString(`<code class="chroma language-` + string(language) + `">`)
+							// the "display" class is used by "js/markup/math.js" to render the code element as a block
+							_, err = w.WriteString(`<code class="chroma language-` + string(language) + ` display">`)
 							if err != nil {
 								return
 							}