Sample: Basic Unity Game

While ModiBuff was tested with two basic prototypes, in Godot and Unity, it doesn't have a single good sample implementation to show basic usage.

This is where this issue and #20 come in.

A demo scene is needed to have ModiBuff on the asset store, and this issue will provide most of it.

**The Game**

The game implementation is supposed to be **very** easy. The only requirement is that ModiBuff.Core is used, and Unity's MonoBehaviours are used.

ModiBuff.Units can also be used, but it's not required. It's up to the developer if they want to implement their own Unit logic.
It's actually a bit preferred to not use ModiBuff.Units, since it comes with a huge implementation baggage, and it's easier to read and understand code, if we don't have to go through all the possible functionalities of the ModiBuff.Units `Unit`.