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/PromiseJobs.cpp

@@ -84,7 +84,6 @@ ThrowCompletionOr<Value> PromiseReactionJob::call()
     // h. If handlerResult is an abrupt completion, then
     if (handler_result.is_abrupt()) {
         vm.clear_exception();
-        vm.stop_unwind();
 
         // i. Let status be Call(promiseCapability.[[Reject]], undefined, « handlerResult.[[Value]] »).
         auto* reject_function = promise_capability.value().reject;
@@ -139,7 +138,6 @@ ThrowCompletionOr<Value> PromiseResolveThenableJob::call()
     // c. If thenCallResult is an abrupt completion, then
     if (then_call_result.is_error()) {
         vm.clear_exception();
-        vm.stop_unwind();
 
         // i. Let status be Call(resolvingFunctions.[[Reject]], undefined, « thenCallResult.[[Value]] »).
         dbgln_if(PROMISE_DEBUG, "[PromiseResolveThenableJob @ {}]: then_call_result is an abrupt completion, calling reject function with value {}", this, *then_call_result.throw_completion().value());