feat(extensions): register commands and providers also for MDX files

[vecerek]

I found that the MDX plugin chose mdx as the language identifier. This PR updates all places where commands/providers are registered to also include mdx files. It also adds an additional activation event for the mdx language.

This PR fixes the issue of auto-completion not working in MDX files (#172). However, syntax highlighting of TypeScript code within JSX blocks still doesn't work. Which part of the code is responsible for the highlighting? I couldn't figure it out just by reading the code and I couldn't make the debugger work, either.

Would you have any pointers for me, @orta?

UPDATE: after playing a bit around with mdx, I noticed that it's also other language code samples that aren't highlighted. I think this indicates that it is the MDX vs code extension that's missing the code highlighting feature.
And indeed, it seems like that extension is having many highlighting bugs, example: mdx-js/mdx-analyzer#209.

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[kamranayub]

It would be awesome to have this; all our new docs are in MDX for Docusaurus. It would definitely enhance the contributor DX to have this working.