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/Interpreter.cpp

@@ -62,10 +62,6 @@ void Interpreter::run(GlobalObject& global_object, const Program& program)
     auto completion = program.execute(*this, global_object);
     vm.set_last_value(Badge<Interpreter> {}, completion.value().value_or(js_undefined()));
 
-    // FIXME: We unconditionally stop the unwind here this should be done using completions leaving
-    //        the VM in a cleaner state after executing. For example it does still store the exception.
-    vm.stop_unwind();
-
     // At this point we may have already run any queued promise jobs via on_call_stack_emptied,
     // in which case this is a no-op.
     vm.run_queued_promise_jobs();