HackStudio: Integate with C++ parser-based autocomplete

By default, C++ auto completion will still be performed by the
lexer-based logic.
However, the parser-based logic can be switched on via the menubar.

Userland/DevTools/HackStudio/LanguageServers/LanguageServer.ipc

@@ -8,4 +8,5 @@ endpoint LanguageServer = 8001
     SetFileContent(String file_name, String content) =|
 
     AutoCompleteSuggestions(String file_name, i32 cursor_line, i32 cursor_column) =|
+    SetAutoCompleteMode(String mode) =|
 }