Apply Godot copyright header only on the platform folders that we maintain upstream.
This lets downstream forks decide what to do with their potential proprietary platforms.
`pre-commit` can be installed with pip, and configured in the Godot repo with
`pre-commit install`. It can then easily be run both locally with
`pre-commit run`, and on CI, in a cross-platform way.
This makes it much easier for contributors to set up pre-commit hooks,
without having to manually copy files to their git folder.
Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>
