refactor: allow some types to be nil

[MariaSolOs]

When updating lsp-format a few days ago, I noticed that I was getting new diagnostics because of the new buffer parameter in on_attach, but the code handles the case in which this parameter is nil, so I'm updating the type accordingly.

While doing that I did the same change to other types, as well as move the method name to a higher scope to replace the raw string references everywhere.

[lukas-reineke]

Thanks

Once I'm done with the new indent blankline version, I'll clean these types up as well. And add LSP check to CI.