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`

doc/ale-hack.txt

@@ -0,0 +1,42 @@
+===============================================================================
+ALE Hack Integration                                         *ale-hack-options*
+
+===============================================================================
+hack                                                            *ale-hack-hack*
+
+g:ale_hack_hhclient_executable                 *g:ale_hack_hhclient_executable*
+                                               *b:ale_hack_hhclient_executable*
+
+  Type: |String|
+  Default: `'hh_client'`
+
+  This variable can be set to use a specific executable to interact with the
+  Hack typechecker.
+
+
+===============================================================================
+hackfmt                                                      *ale-hack-hackfmt*
+
+g:ale_hack_hackfmt_options                         *g:ale_hack_hackfmt_options*
+                                                   *b:ale_hack_hackfmt_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be set to pass additional options to the hackfmt fixer.
+
+
+===============================================================================
+hhast                                                          *ale-hack-hhast*
+
+g:ale_hack_hhast_executable                       *g:ale_hack_hhast_executable*
+                                                  *b:ale_hack_hhast_executable*
+
+  Type: |String|
+  Default: `'vendor/bin/hhast-lint'`
+
+  This variable can be set to use a specific executable to interact with the
+  Hack typechecker.
+
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: