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/Tests/labels.js

@@ -1,4 +1,4 @@
-test("labeled plain scope", () => {
+test("labelled plain scope", () => {
     notused: test: alsonotused: {
         let o = 1;
         expect(o).toBe(1);
@@ -16,7 +16,7 @@ test("break on plain scope from inner scope", () => {
     }
 });
 
-test("labeled for loop with break", () => {
+test("labelled for loop with break", () => {
     let counter = 0;
     notused: outer: alsonotused: for (a of [1, 2, 3]) {
         for (b of [4, 5, 6]) {
@@ -27,7 +27,7 @@ test("labeled for loop with break", () => {
     expect(counter).toBe(4);
 });
 
-test("labeled for loop with continue", () => {
+test("labelled for loop with continue", () => {
     let counter = 0;
     notused: outer: alsonotused: for (a of [1, 2, 3]) {
         for (b of [4, 5, 6]) {
@@ -38,6 +38,32 @@ test("labeled for loop with continue", () => {
     expect(counter).toBe(6);
 });
 
+test("continue label statement is not an iteration statement", () => {
+    expect(() =>
+        eval(`
+outer: outer2: {
+    for (;;) {
+        continue outer;
+    }
+}`)
+    ).toThrowWithMessage(
+        SyntaxError,
+        "labelled continue statement cannot use non iterating statement (line: 4, column: 18)"
+    );
+
+    expect(() =>
+        eval(`
+for (;;) {
+    outer: outer2: {
+        continue outer;
+    }
+}`)
+    ).toThrowWithMessage(
+        SyntaxError,
+        "labelled continue statement cannot use non iterating statement (line: 4, column: 18)"
+    );
+});
+
 test("break on try catch statement", () => {
     let entered = false;
     label1: label2: label3: try {