[Feature proposal] 3D support

[NiclasEriksen]

Inspired by this pull request for the original simulation:
hunar4321/particle-life#40

It's obviously a performance hit, but the complexity involved is negligible. Godot can use the same Viewport for both modes, swapping between two cameras and WorldEnvironment's.

This can be implemented in a separate GDNative library, while the Godot project remains undivided, and they can share rules.