Multigame stage1a

[JeffML]

This encapsulates all game-related methods and state to Game.ts. Constants and other shared state moved to types.js.

This is a backward-compatible refactoring of Chess.js; Chess methods now delegate to a Game instance where warranted.

If this PR is accepted, then I can proceed to Stage 2: multi-game PGN support for chess.js

[JeffML]

#561

[neofight78]

I've not looked at it in detail yet but these are my initial thoughts:

• types.ts - Bit of a nit pick, but this has constants and functions too. I'd call it common.ts or something more generic. But the idea is good / essential. I've had to do the same when work on my RAV POC.
• Move.ts - Yep, good call to pull this out into it's own file. I'd prefer to keep the filenames all lowercase though (sorry, another nitpick).
• Game.ts - This one I'm not so sure about. Keeping the Chess class as essentially the game object and splitting out board state, and adding something new to represent a PGN feels like a more natural direction to me.