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/hack/hack.vim

@@ -0,0 +1,27 @@
+" Author: Fred Emmott <fe@fb.com>
+" Description: Hack support via `hack lsp`
+
+call ale#Set('hack_hhclient_executable', 'hh_client')
+
+function! ale_linters#hack#hack#GetProjectRoot(buffer) abort
+    let l:hhconfig = ale#path#FindNearestFile(a:buffer, '.hhconfig')
+    return !empty(l:hhconfig) ? fnamemodify(l:hhconfig, ':h') : ''
+endfunction
+
+function! ale_linters#hack#hack#GetExecutable(buffer) abort
+    return ale#Var(a:buffer, 'hack_hhclient_executable')
+endfunction
+
+
+function! ale_linters#hack#hack#GetCommand(buffer) abort
+    let l:executable = ale_linters#hack#hack#GetExecutable(a:buffer)
+    return ale#Escape(l:executable).' lsp --from vim-ale'
+endfunction
+
+call ale#linter#Define('hack', {
+\   'name': 'hack',
+\   'lsp': 'stdio',
+\   'executable_callback': 'ale_linters#hack#hack#GetExecutable',
+\   'command_callback': 'ale_linters#hack#hack#GetCommand',
+\   'project_root_callback': 'ale_linters#hack#hack#GetProjectRoot',
+\})