Optimize Asset Lib

The [asset lib](https://github.com/VisualPinball/VisualPinball.Unity.AssetLibrary), along with its references and history, is getting too big. We need to take better care of optimizing assets, and store source files elsewhere than in the repo itself. A first step into that direction is the [style guide](https://docs.visualpinball.org/creators-guide/editor/asset-library-styleguide.html), but we need a new repo and start porting assets over in an optimized way.

So, the first step would be to create a new repo and then start porting things over. Unfortunately, the name `VisualPinball.Unity.AssetLibrary` is already taken, so the *very* first step would be to rename the old repo into something like `VisualPinball.Unity.AssetLibrary.Legacy` and then start from fresh with `VisualPinball.Unity.AssetLibrary`.

Whether the new repo should be LFS or not is another question, since LFS can have significant costs. But given we'd only push optimized assets to the repo, going for LFS and switching to self-hosted in case it gets too expensive (which it really shouldn't), might be the best path forward.

Solves #534.