[lsp] Add Swift / SourceKit-LSP support

[namolnad]

Swift LSP Support

This adds LSP support for Swift via the recently announced/open-source SourceKit-LSP.

Addresses #2147 (Thanks @TaDaa for the initial work on the fork — this is based on your original work.)

Tasks:

• Add LSP support for Swift via SourceKit-LSP
• Add tests

[w0rp]

Add in a test for the linter executable and command. Then we can test that the linter will be loaded okay. See :help ale-tests and look at some existing tests for examples. Otherwise, this looks pretty good to me. 👍

[namolnad]

Thanks for the feedback @w0rp! I updated with some additional tests based off some existing examples.

(Coincidentally after trying to run :help ale-tests I also realized I needed to generate help tags for my plugins and used your example in the main README to do that — so thanks for that as well!)

[w0rp]

Cheers! 🍻

[w0rp]

One thing I forgot, which you can add now, is add the new linter to the list of supported tools in Markdown and in the Vim help file.