add project_root_callback for tsserver (#2290)

* add project_root_callback for tsserver
* add tests for tsserver project root
* Update test/command_callback/test_javascript_tsserver_command_callback.vader

autoload/ale/handlers/tsserver.vim

@@ -0,0 +1,8 @@
+" Author: Derek Sifford <dereksifford@gmail.com>
+" Description: Handlers for tsserver
+
+function! ale#handlers#tsserver#GetProjectRoot(buffer) abort
+    let l:tsconfig_file = ale#path#FindNearestFile(a:buffer, 'tsconfig.json')
+
+    return !empty(l:tsconfig_file) ? fnamemodify(l:tsconfig_file, ':h') : ''
+endfunction