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/fixers/test_hackfmt_fixer_callback.vader

@@ -1,10 +1,10 @@
 Before:
-  Save g:ale_php_hackfmt_executable
-  Save g:ale_php_hackfmt_options
+  Save g:ale_hack_hackfmt_executable
+  Save g:ale_hack_hackfmt_options
 
   " Use an invalid global executable, so we don't match it.
-  let g:ale_php_hackfmt_executable = 'xxxinvalid'
-  let g:ale_php_hackfmt_options = ''
+  let g:ale_hack_hackfmt_executable = 'xxxinvalid'
+  let g:ale_hack_hackfmt_options = ''
 
   call ale#test#SetDirectory('/testplugin/test/fixers')
 
@@ -14,7 +14,7 @@ After:
   call ale#test#RestoreDirectory()
 
 Execute(The hackfmt callback should return the correct default values):
-  call ale#test#SetFilename('../hack_files/testfile.php')
+  call ale#test#SetFilename('../hack_files/testfile.hack')
 
   AssertEqual
   \ {
@@ -25,8 +25,8 @@ Execute(The hackfmt callback should return the correct default values):
   \ ale#fixers#hackfmt#Fix(bufnr(''))
 
 Execute(The hackfmt callback should include custom hackfmt options):
-  let g:ale_php_hackfmt_options = "--some-option"
-  call ale#test#SetFilename('../hack_files/testfile.php')
+  let g:ale_hack_hackfmt_options = "--some-option"
+  call ale#test#SetFilename('../hack_files/testfile.hack')
 
   AssertEqual
   \ {

test/hack_files/testfile.php

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