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

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

test/command_callback/test_swift_sourcekitlsp_command_callbacks.vader

@@ -0,0 +1,21 @@
+Before:
+  call ale#assert#SetUpLinterTest('swift', 'sourcekitlsp')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The default executable path should be correct):
+  call ale#test#SetFilename('../swift-test-files/swift-package-project/src/folder/dummy.swift')
+
+  AssertLinter 'sourcekit-lsp', ale#Escape('sourcekit-lsp')
+
+Execute(Should let users configure a global executable and override local paths):
+  call ale#test#SetFilename('../swift-test-files/swift-package-project/src/folder/dummy.swift')
+
+  let g:ale_sourcekit_lsp_executable = '/path/to/custom/sourcekitlsp'
+
+  AssertLinter '/path/to/custom/sourcekitlsp',
+  \ ale#Escape('/path/to/custom/sourcekitlsp')
+
+Execute(The language should be correct):
+  AssertLSPLanguage 'swift'

test/test_swift_find_project_root.vader

@@ -0,0 +1,18 @@
+Before:
+  call ale#test#SetDirectory('/testplugin/test')
+
+After:
+  call ale#test#RestoreDirectory()
+
+Execute(Detect root of Swift project with Package.swift correctly):
+  call ale#test#SetFilename('swift-test-files/swift-package-project/src/folder/dummy.swift')
+  AssertEqual
+  \ ale#path#Simplify(g:dir . '/swift-test-files/swift-package-project'),
+  \ ale#swift#FindProjectRoot(bufnr(''))
+
+Execute(Detect no root in case of non-Package.swift project):
+  call ale#test#SetFilename('swift-test-files/non-swift-package-project/src/folder/dummy.swift')
+  AssertEqual
+  \ '',
+  \ ale#swift#FindProjectRoot(bufnr(''))
+