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

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

autoload/ale/swift.vim

@@ -0,0 +1,13 @@
+" Author: Dan Loman <https://github.com/namolnad>
+" Description: Functions for integrating with Swift tools
+
+" Find the nearest dir containing a Package.swift file and assume it is the root of the Swift project.
+function! ale#swift#FindProjectRoot(buffer) abort
+    let l:swift_config = ale#path#FindNearestFile(a:buffer, 'Package.swift')
+
+    if !empty(l:swift_config)
+        return fnamemodify(l:swift_config, ':h')
+    endif
+
+    return ''
+endfunction