feat: add document_formatting_provider to lsp

[benjaminjellis]

Hi, first off loon is really cool and checks a huge number of boxes for me so thanks for you work so far!

I'm not sure if you're open to contributions so feel free to close this if you aren't.

But when I was looking at the docs and trying to set up auto-format in neovim I would get the following error:

method textDocument/formatting is not supported by any of the servers registered for the current buffer

After some poking around at tower-lsp it seemed fairly trivial to add so here it is. Let me know if you'd need any changes to merge.

Thanks!

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