Split Hack out from PHP, modernize

fixes #1738

- Replace previous `hh_client` usage with LSP client
- Add `HHAST` linter
- Split Hack from PHP: Hack is increasingly diverging from PHP:
  - Hack tools do not understand PHP
  - Most PHP tools do not handle Hack code well (including vim's syntax
    highightling files)
  - http://github.com/hhvm/vim-hack now sets filetype to `hack`

test/hack_files/testfile.php

@@ -0,0 +1,4 @@
+<?hh // strict
+
+async function foo(): Awaitable<void> {
+}