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/Utilities/js.cpp

@@ -795,7 +795,6 @@ static void print_value(JS::Value value, HashTable<JS::Object*>& seen_objects)
         if (prototype_or_error.has_value() && prototype_or_error.value() == object.global_object().error_prototype())
             return print_error(object, seen_objects);
         vm->clear_exception();
-        vm->stop_unwind();
 
         if (is<JS::RegExpObject>(object))
             return print_regexp_object(object, seen_objects);