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/LibWeb/WebAssembly/WebAssemblyObject.cpp

@@ -162,7 +162,6 @@ JS_DEFINE_NATIVE_FUNCTION(WebAssemblyObject::compile)
     if (buffer_or_error.is_error()) {
         rejection_value = *buffer_or_error.throw_completion().value();
         vm.clear_exception();
-        vm.stop_unwind();
     }
     auto promise = JS::Promise::create(global_object);
     if (!rejection_value.is_empty()) {
@@ -327,7 +326,6 @@ JS_DEFINE_NATIVE_FUNCTION(WebAssemblyObject::instantiate)
     if (buffer_or_error.is_error()) {
         auto rejection_value = *buffer_or_error.throw_completion().value();
         vm.clear_exception();
-        vm.stop_unwind();
         promise->reject(rejection_value);
         return promise;
     }