LibJS: Stop executing successful regex if it's past the end of the input

If the regex always matches the input, even if it's past the end, then
we need to stop execution of the regex when it's past the end. This
corresponds to step 13.a and prevents it from infinitely looping.

Reduced from: d98672060f/packages/react-i18n/src/utilities/money.ts (L10-L14)

Libraries/LibJS/Runtime/RegExpPrototype.cpp

@@ -224,7 +224,7 @@ static ThrowCompletionOr<Value> regexp_builtin_exec(VM& vm, RegExpObject& regexp
     result = regex.match(string.view());
 
     // 13.d and 13.a
-    if (!result.success) {
+    if (!result.success || last_index > string.length_in_code_units()) {
         // 13.d.i, 13.a.i
         if (sticky || global)
             TRY(regexp_object.set(vm.names.lastIndex, Value(0), Object::ShouldThrowExceptions::Yes));

Libraries/LibJS/Tests/builtins/RegExp/RegExp.js

@@ -66,3 +66,9 @@ test("Incorrectly escaped code units not converted to invalid patterns", () => {
     expect(re.test("⫀")).toBeTrue();
     expect(re.test("\\u2abe")).toBeFalse(); // ⫀ is \u2abe
 });
+
+test("regexp that always matches stops matching if it's past the end of the string instead of infinitely looping", () => {
+    const re = new RegExp("[\u200E]*", "gu");
+    expect("whf".match(re)).toEqual(["", "", "", ""]);
+    expect(re.lastIndex).toBe(0);
+});