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/PromiseReaction.h

@@ -26,7 +26,6 @@ struct PromiseCapability {
         /* 1. If value is an abrupt completion, then */                                                                 \
         if (_temporary_try_or_reject_result.is_error()) {                                                               \
             vm.clear_exception();                                                                                       \
-            vm.stop_unwind();                                                                                           \
                                                                                                                         \
             /* a. Perform ? Call(capability.[[Reject]], undefined, « value.[[Value]] »). */                           \
             TRY(vm.call(*capability.reject, js_undefined(), *_temporary_try_or_reject_result.release_error().value())); \