AK: Make it possible to not using AK classes into the global namespace

This patch adds the `USING_AK_GLOBALLY` macro which is enabled by
default, but can be overridden by build flags.

This is a step towards integrating Jakt and AK types.

AK/NonnullRefPtr.h

@@ -256,6 +256,8 @@ struct Traits<NonnullRefPtr<T>> : public GenericTraits<NonnullRefPtr<T>> {
     static bool equals(NonnullRefPtr<T> const& a, NonnullRefPtr<T> const& b) { return a.ptr() == b.ptr(); }
 };
 
+#if USING_AK_GLOBALLY
 using AK::adopt_ref;
 using AK::make_ref_counted;
 using AK::NonnullRefPtr;
+#endif