Add GH action to run gdlint on PRs and branches

[Jibbajabbafic]

This python module contains tools to lint and format GDscript files to make sure they comply with the style guide:
https://pypi.org/project/gdtoolkit/

It would be useful to run these checks with Github Actions on all PRs and the main branch to make sure the style is consistent.

We could even auto format all scripts in PRs and then commit it with something like this, but it might not always be right so it would be nice to have a choice if the change gets commited or not: https://github.com/marketplace/actions/add-commit