Improve playerstate situation

[rom1504]

1. Define cleanly what player state object should contain (in the readme)
2. Add a function that check that the object fit this spec, call it in the simulate function (with an option not to call it for case where it could affect speed like pathfinder)
3. Move translation from mineflayer to prismarine-physics in mineflayer, to avoid having mineflayer specific code here