LibJS: Implement RegExp.prototype.toString() as standalone function

This should not just inherit Object.prototype.toString() (and override
Object::to_string()) but be its own function, i.e.
'RegExp.prototype.toString !== Object.prototype.toString'.

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

@@ -0,0 +1,5 @@
+test("basic functionality", () => {
+    expect(RegExp.prototype.toString).toHaveLength(0);
+
+    expect(/test/g.toString()).toBe("/test/g");
+});