LibRegex: Allow references to capture groups that aren't parsed yet

This only applies to the ECMA262 parser.
This behaviour is an ECMA262-specific quirk, such references always
generate zero-length matches (even on subsequent passes).
Also adds a test in LibJS's test suite.

Fixes #6039.

Userland/Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.exec.js

@@ -66,3 +66,15 @@ test("not matching", () => {
 
     expect(res).toBe(null);
 });
+
+// Backreference to a group not yet parsed: #6039
+test("Future group backreference, #6039", () => {
+    let re = /(\3)(\1)(a)/;
+    let result = re.exec("cat");
+    expect(result.length).toBe(4);
+    expect(result[0]).toBe("a");
+    expect(result[1]).toBe("");
+    expect(result[2]).toBe("");
+    expect(result[3]).toBe("a");
+    expect(result.index).toBe(1);
+});