Refactor render system (#32492)

There were too many patches to the Render system, it's really difficult
to make further improvements.

This PR clears the legacy problems and fix TODOs.

1. Rename `RenderContext.Type` to `RenderContext.MarkupType` to clarify
its usage.
2. Use `ContentMode` to replace `meta["mode"]` and `IsWiki`, to clarify
the rendering behaviors.
3. Use "wiki" mode instead of "mode=gfm + wiki=true"
4. Merge `renderByType` and `renderByFile`
5. Add more comments

----

The problem of "mode=document": in many cases it is not set, so many
non-comment places use comment's hard line break incorrectly

web_src/js/features/comp/ComboMarkdownEditor.ts

@@ -74,7 +74,6 @@ export class ComboMarkdownEditor {
   previewUrl: string;
   previewContext: string;
   previewMode: string;
-  previewWiki: boolean;
 
   constructor(container, options = {}) {
     container._giteaComboMarkdownEditor = this;
@@ -213,13 +212,11 @@ export class ComboMarkdownEditor {
     this.previewUrl = this.tabPreviewer.getAttribute('data-preview-url');
     this.previewContext = this.tabPreviewer.getAttribute('data-preview-context');
     this.previewMode = this.options.previewMode ?? 'comment';
-    this.previewWiki = this.options.previewWiki ?? false;
     this.tabPreviewer.addEventListener('click', async () => {
       const formData = new FormData();
       formData.append('mode', this.previewMode);
       formData.append('context', this.previewContext);
       formData.append('text', this.value());
-      formData.append('wiki', String(this.previewWiki));
       const response = await POST(this.previewUrl, {data: formData});
       const data = await response.text();
       renderPreviewPanelContent($(panelPreviewer), data);