Refactor the physics engine to a separate package

[SiiMeR]

Since the custom physics engine using raycasts works really well, it would be useful to make this a separate package. Then it could be easily included in new projects just by adding it through Unity package manager.

This refactor would also remove the coupling of code that exists currently, which would mean less headaches further down the road.

Progress

• Create a new project and repository to host the engine
• Determine the level of refactoring needed
• Refactor the code inside BallTale so it is independent
• Move engine to new project
• Write tests to verify that everything works as expected
• Configure package metadata (see also https://gist.github.com/LotteMakesStuff/6e02e0ea303030517a071a1c81eb016e)
• Remove engine code from BallTale
• Add in the engine through the package manager
• Verify that everything works as expected

[SiiMeR]

Custom Unity Package Manager support was added in 2018.3: https://forum.unity.com/threads/git-support-on-package-manager.573673/