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`

ale_linters/php/hack.vim

@@ -1,28 +0,0 @@
-" Author: Zefei Xuan <https://github.com/zefei>
-" Description: Hack type checking (http://hacklang.org/)
-
-function! ale_linters#php#hack#Handle(buffer, lines) abort
-    let l:pattern = '^\(.*\):\(\d\+\):\(\d\+\),\(\d\+\): \(.\+])\)$'
-    let l:output = []
-
-    for l:match in ale#util#GetMatches(a:lines, l:pattern)
-        if a:buffer != bufnr(l:match[1])
-            continue
-        endif
-
-        call add(l:output, {
-        \   'lnum': l:match[2] + 0,
-        \   'col': l:match[3] + 0,
-        \   'text': l:match[5],
-        \})
-    endfor
-
-    return l:output
-endfunction
-
-call ale#linter#Define('php', {
-\   'name': 'hack',
-\   'executable': 'hh_client',
-\   'command': 'hh_client --retries 0 --retry-if-init false',
-\   'callback': 'ale_linters#php#hack#Handle',
-\})