Deprecate preview support for markdown files

[msujew]

Since we're now deprecating the Git extension (#12745) in favor of using VS Code's builtins, I'm wondering whether it makes sense to remove the markdown preview feature from the @theia/preview package. IMO it's a worse version of the vscode.markdown builtin extension and bloats the bundle due to the inclusion of highlight.js (and its 1.3mb minified bundle size).

[msujew]

This has been discussed in last weeks dev call. We will likely deprecate/remove the feature after sending out an info about this via the Theia mailing list.

I aim to remove the related code for the August (1.53) release.

[zhouxinghong]

@msujew I've asked VSCode many times about these problems but they refused to deal with them, besides VSCode is very bloated and slow now, so when I found Theia I decided to abandon VSCode completely and move to Theia, I hope that the birth of the Theia project means that it's an enhancement and improvement of VSCode, at least in some ways, it should have its own unique features. I hope that the Theia project is meant to be an enhanced and improved version of VSCode, at least in some ways.
I hope you will consider these suggestions.

Thank you very much for your hard work! 👍😜👏

[zhouxinghong]

The preview button with the arrow pointing to it has very complete support for audio and video previews, and the previews are much better than the vscode Markdown previews, so please keep this feature. Also, vscode Markdown is a feature-crippled presence, consider removing the vscode Markdown preview.

[zhouxinghong]

Version 1.53.200
Windows 10 19044

Hello my friend! Please allow users to remove unnecessary buttons.
For example, use the button pointed by the red scissors:

Because it calls the Markdown preview function that comes with VSCode, after a long period of testing and use, it is found that the Markdown preview function that comes with VSCode is a broken, outdated and incomplete existence, so please delete it.

Please remove this old right-click menu item.
Because it calls the Markdown preview function that comes with VSCode, after a long period of testing and use, it is found that the Markdown preview function that comes with VSCode is a broken, outdated and incomplete existence, so please delete it.

Please remove this old preview option.
Because it calls the Markdown preview function that comes with VSCode, after a long period of testing and use, it is found that the Markdown preview function that comes with VSCode is a broken, outdated and incomplete existence, so please delete it.

Version 1.53.200
Windows 10 19044