Add linter for haskell-language-server

The patch adds a new linter for haskell-language-server (hls). hls is
the integration point of haskell-ide-engine (hie) and ghcide.

test/command_callback/test_haskell_hls_callbacks.vader

@@ -0,0 +1,27 @@
+Before:
+  call ale#assert#SetUpLinterTest('haskell', 'hls')
+
+  Save &filetype
+  let &filetype = 'haskell'
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The language string should be correct):
+  AssertLSPLanguage 'haskell'
+
+Execute(The default executable should be correct):
+  AssertLinter 'haskell-language-server-wrapper',
+  \ ale#Escape('haskell-language-server-wrapper') . ' --lsp'
+
+Execute(The project root should be detected correctly):
+  AssertLSPProject g:dir
+
+  call ale#test#SetFilename('hls_paths/file.hs')
+
+  AssertLSPProject ale#path#Simplify(g:dir . '/hls_paths')
+
+Execute(The executable should be configurable):
+  let g:ale_haskell_hls_executable = 'foobar'
+
+  AssertLinter 'foobar', ale#Escape('foobar') . ' --lsp'