Rename demo folder to project for generalizing the template usecase

[paddy-exe]

Renames all references to the demo folder and changes it to "project"

As discussed in the last GDExtension meeting, the demo folder should be renamed to project for generalized usage of the template (addon extensions as well as custom game-based extensions)

[Ivorforce]

I'm not 100% sold on "project" for the folder name, because it's a little ambiguous what "project" means (because the parent folder of "project" is already a "project").

The least ambiguous thing I can think of would be "Godot project" but calling it "godot-project" or "godot" doesn't feel right either...

[paddy-exe]

I'm not 100% sold on "project" for the folder name, because it's a little ambiguous what "project" means (because the parent folder of "project" is already a "project").

The least ambiguous thing I can think of would be "Godot project" but calling it "godot-project" or "godot" doesn't feel right either...

Hmmm, yeah understandable. Maybe extension-demo? That way it would be clear that it could be used generally but also clear enough that it is where the demo project is?

[dsnopek]

Thanks!

Unfortunately, I can't think of any better directory name than "project". If we're catering to both use cases, the name needs to makes sense when:

1. This is a reusable GDExtension and the Godot project is a demo, as well as when...
2. This is a "game logic" GDExtension for one specific game, and the Godot project is the game

"project" isn't perfect, but it does work in both those cases