LibJS: Make RegExp.prototype.flags spec-compliant

This should be using the individual flag boolean properties rather than
the [[OriginalFlags]] internal slot.
Use an enumerator macro here for brevity, this will be useful for other
things as well. :^)

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

@@ -0,0 +1,10 @@
+test("basic functionality", () => {
+    expect(/foo/.flags).toBe("");
+    expect(/foo/g.flags).toBe("g");
+    expect(/foo/i.flags).toBe("i");
+    expect(/foo/m.flags).toBe("m");
+    expect(/foo/s.flags).toBe("s");
+    expect(/foo/u.flags).toBe("u");
+    expect(/foo/y.flags).toBe("y");
+    expect(/foo/sgimyu.flags).toBe("gimsuy");
+});