Add autoimport and rename support for langservers

[jeremija]

This PR contains the following updates:

• ALERename command now works with langservers (previously only with tsserver).
• Autoimport functionality now works with langservers (previously only with tsserver).

I've been using both with gopls for a few months and it's been working pretty well.

Closes #2976

[w0rp]

Sorry for the delay. I'll merge this now.

I went looking for this as I want to add a command to ALE that takes advantage of completions with imports so we can find the word under the cursor and insert the import line that we find.

[w0rp]

Cheers! 🍻

[w0rp]

I'll add a new import setting and deprecate the old one, as it mentions tsserver in the name.

[w0rp]

@jeremija I'm going to disable completion results that add import lines by default, which will match what ALE does with tsserver. You can get a lot of surprising completion results that way, and it will take longer to search a whole project for imports instead of just the current file, for many servers. Use let g:ale_completion_autoimport = 1 after I push my patch in a moment to turn it on.