[lsp] Add Swift / SourceKit-LSP support (#2420)

* [linter] Add LSP support for Swift via sourcekit-lsp

ale_linters/swift/sourcekitlsp.vim

@@ -0,0 +1,13 @@
+" Author: Dan Loman <https://github.com/namolnad>
+" Description: Support for sourcekit-lsp https://github.com/apple/sourcekit-lsp
+
+call ale#Set('sourcekit_lsp_executable', 'sourcekit-lsp')
+
+call ale#linter#Define('swift', {
+\   'name': 'sourcekitlsp',
+\   'lsp': 'stdio',
+\   'executable': {b -> ale#Var(b, 'sourcekit_lsp_executable')},
+\   'command': '%e',
+\   'project_root': function('ale#swift#FindProjectRoot'),
+\   'language': 'swift',
+\})