Introduce GitHub markdown editor, keep EasyMDE as fallback (#23876)
The first step of the plan
* #23290
Thanks to @silverwind for the first try in #15394 . Close #10729 and a
lot of related issues.
The EasyMDE is not removed, now it works as a fallback, users can switch
between these two editors.
Editor list:
* Issue / PR comment
* Issue / PR comment edit
* Issue / PR comment quote reply
* PR diff view, inline comment
* PR diff view, inline comment edit
* PR diff view, inline comment quote reply
* Release editor
* Wiki editor
Some editors have attached dropzone
Screenshots:
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Co-authored-by: silverwind <me@silverwind.io>
