LibJS: Implement Intl.Locale.prototype.toString()

This isn't particularly testable yet without the Intl.Locale constructor
but having this defined will make testing the constructor possible. So
more specific tests for this prototype will come later.

Userland/Libraries/LibJS/Tests/builtins/Intl/Locale/Locale.prototype.toString.js

@@ -0,0 +1,3 @@
+test("length is 0", () => {
+    expect(Intl.Locale.prototype.toString).toHaveLength(0);
+});