Generate `gdj` and `kt` files from within the godot editor

[chippmann]

With https://github.com/utopia-rise/godot-kotlin-jvm/pull/441 we introduced the concept of "registration files".

We now need a way to generate a gdj and a kt when the user creates a new script inside the editor.

One way could be with a custom resource saver where we not only save a gdj file, but also create a corresponding kt file in the root package.

[CedNaru]

I think this proposal has to be paired with #500. It would make the implementation much easier if the .gdj files could be freely placed inside the godot project. The .kt and .gdj would then both be created next to each other. No need for custom C++ logic to place them in the correct directory (directory only known by our gradle plugin by the way).