Added flatworld envs (dense)

[nicomon24]

Adding to environments:

• MineRLNavigateDenseFlat-v0: flat world, dense rewards and goal around the compass targat at distance 8
• MineRLNavigateDenseFlatDeterministic-v0: flat world, dense rewards, goal is exactly pointed by the compass. Used for testing and solvable using just the compass angle.

Could be extended also in the sparse setting for testing.