Adding support for MarkdownClientCapabilities

[MartinK07]

With LSP 3.16 an option was introduced in which clients can specify the markdown parser that the server should use.
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#markdownClientCapabilities

Having the option to specify the flavor of Markdown to generate would be a great way to resolve differences between GFM (GitHub Flavoured Markdown) and Python-Markdown.

This would likely resolve issues for currently broken rendering in LSP clients such as Sublime-LSP:
python-lsp/docstring-to-markdown#36

[krassowski]

I would be in favour of handling the MarkdownClientCapabilities and passing this info down to docstring-to-markdown (which would also need to be modified to accept these too); I am happy to help review PRs if anyone is interested in contributing this.