Set up the framework so users can have it as a git submodule

[NathanLovato]

From https://www.reddit.com/r/godot/comments/ffoyqr/trying_out_gdquests_steering_framework/

I'd personally like if the repo was better setup to be a submodule. There could be a core repo and a demo repo, the demo repo could use the core repo as a submodule. If thats too hardcore for your audience, can package releases that have them all zipped up.