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/Parser.h

@@ -120,7 +120,7 @@ public:
     NonnullRefPtr<ExportStatement> parse_export_statement(Program& program);
 
     RefPtr<FunctionExpression> try_parse_arrow_function_expression(bool expect_parens, bool is_async = false);
-    RefPtr<Statement> try_parse_labelled_statement(AllowLabelledFunction allow_function);
+    RefPtr<LabelledStatement> try_parse_labelled_statement(AllowLabelledFunction allow_function);
     RefPtr<MetaProperty> try_parse_new_target_expression();
     RefPtr<MetaProperty> try_parse_import_meta_expression();
     NonnullRefPtr<ImportCall> parse_import_call();