LibJS: Replace the custom unwind mechanism with completions :^)

This includes:

- Parsing proper LabelledStatements with try_parse_labelled_statement()
- Removing LabelableStatement
- Implementing the LoopEvaluation semantics via loop_evaluation() in
  each IterationStatement subclass; and IterationStatement evaluation
  via {For,ForIn,ForOf,ForAwaitOf,While,DoWhile}Statement::execute()
- Updating ReturnStatement, BreakStatement and ContinueStatement to
  return the appropriate completion types
- Basically reimplementing TryStatement and SwitchStatement according to
  the spec, using completions
- Honoring result completion types in AsyncBlockStart and
  OrdinaryCallEvaluateBody
- Removing any uses of the VM unwind mechanism - most importantly,
  VM::throw_exception() now exclusively sets an exception and no longer
  triggers any unwinding mechanism.
  However, we already did a good job updating all of LibWeb and userland
  applications to not use it, and the few remaining uses elsewhere don't
  rely on unwinding AFAICT.

Userland/Libraries/LibJS/Runtime/AsyncFunctionDriverWrapper.cpp

@@ -39,7 +39,6 @@ ThrowCompletionOr<Value> AsyncFunctionDriverWrapper::react_to_async_task_complet
     if (generator_result.is_throw_completion()) {
         VERIFY(generator_result.throw_completion().type() == Completion::Type::Throw);
         vm.clear_exception();
-        vm.stop_unwind();
         auto promise = Promise::create(global_object);
         promise->reject(*generator_result.throw_completion().value());
         return promise;