Add root piece getters to Lua API

[sprunk]

No description provided.

[lhog]

Hmm isn't root piece always [0] ?

[sprunk]

Game devs don't know that. I had a strong suspicion this is the case but there is no foolproof way to learn this other than reading engine source. I've seen a chunk of lua code that gets all pieces and checks them one by one to find the root, which is what prompted this PR (since I wanted to replace it with something simpler and realized there isn't any).

Some sort of Engine.rootPieceIndex constant would also be fine if we anticipate we will never be using formats that have the root piece elsewhere (it may well be a good assumption since the numbers can be remapped, but idk).

[lhog]

Root piece is always zero, no matter how pieces arranged in the model.
But yeah, I guess it can't hurt.