Add a minimal CI

[jacobsky]

Currently there are no CI or Publishing actions. While this is tentatively "fixed", I think it would be worthwhile to investigate and work in a more full featured CI so that we can make sure we understand the health of the project.

I'm also less likely to miss something silly when merging PRs.

[jacobsky]

As part of the CI, it would be very helpful is any PRs can open the project and upload some screenshots of the various screens such.

This can be accomplished with

var image = get_viewport().get_texture().get_data()
image.save_png("screenshot.png")