Use swift's language server

[merisbahti]

Apple has released a new language server protocol implementation, is this something we could use in this project?

https://github.com/apple/sourcekit-lsp

[keith]

We could definitely add configuration for LSP plugins that supported sourcekit-LSP here, just like we have for running swiftlint. But I also think we could add many of those to the upstream LSP plugins as well. I'm currently using it with vim-LSP which that project has example setup instructions for https://github.com/apple/sourcekit-lsp/blob/master/Editors/README.md#vim-8