Demos / Scenes as Sub-Modules

Is your feature request related to a problem? Please describe.

Currently these folders behave like submodules and are not included in release builds. By organizing them into a separate repo we should gain a few benefits:

• development discussion specifically related to demo and test scene creation can be consolidated separately, in the same way as gaea-docs.
• the new repo can be specifically added to the Godot Asset Library as a separate item.

Describe the solution you'd like

A new repo gaea-demos has already been created, and the work of using it for submodules for both the 1.X and 2.0 branches is already complete.

Describe alternatives you've considered

These demos could be included in release builds, as an alternative. It's worth mentioning that the Godot Asset Library does not support sub-modules.

Is there a reason this should be core and not a project-specific feature?

This is an inherently core-scoped decision.