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.

[msujew]

@zhouxinghong I appreciate the feedback. While I cannot entirely agree that the markdown preview by VS Code is really broken or outdated (and least not more broken or outdated than our own, 6 year old implementation), I can see the benefit of us delivering our own markdown preview feature.

However, we're dealing with a lot of code to maintain in our codebase already (250.000+ LoC), so every additional feature actually puts a lot of maintenance burden on our end. In fact, I don't think we did a good job of maintaining the feature at all. If we can offload this effort to the VS Code team (a much larger team than ours), we try to do that so we can focus on the more important features.