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:
<details>
![image](https://user-images.githubusercontent.com/2114189/229363558-7e44dcd4-fb6d-48a0-92f8-bd12f57bb0a0.png)
![image](https://user-images.githubusercontent.com/2114189/229363566-781489c8-5306-4347-9714-d71af5d5b0b1.png)
![image](https://user-images.githubusercontent.com/2114189/229363771-1717bf5c-0f2a-4fc2-ba84-4f5b2a343a11.png)
![image](https://user-images.githubusercontent.com/2114189/229363793-ad362d0f-a045-47bd-8f9d-05a9a842bb39.png)
</details>
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Co-authored-by: silverwind <me@silverwind.io>