LibJS: Don't assume match for each capture group in RegExp.prototype.exec()

This was not implementing the following part of the spec correctly:

    27. For each integer i such that i ≥ 1 and i ≤ n, do
        a. Let captureI be ith element of r's captures List.
        b. If captureI is undefined, let capturedValue be undefined.

Expecting a capture group match to exist for each of the RegExp's
capture groups would assert in Vector's operator[] if that's not the
case, for example:

    /(foo)(bar)?/.exec("foo")

Append undefined instead.

Fixes #5256.

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

@@ -18,6 +18,16 @@ test("basic unnamed captures", () => {
     expect(res[1]).toBe("ooooo");
     expect(res.groups).toBe(undefined);
     expect(res.index).toBe(0);
+
+    re = /(foo)(bar)?/;
+    res = re.exec("foo");
+
+    expect(res.length).toBe(3);
+    expect(res[0]).toBe("foo");
+    expect(res[1]).toBe("foo");
+    expect(res[2]).toBe(undefined);
+    expect(res.groups).toBe(undefined);
+    expect(res.index).toBe(0);
 });
 
 test("basic named captures", () => {