updates for basic support of Gymnasium (+ updates to rendering approach)

[SimonOuellette35]

• if you're interested, here is a pull request that provides basic support for Gymnasium. I had to modify how rendering works by leveraging the HumanRendering wrapper in Gymnasium, which in turn forced me to somewhat redesign how selection of rendering mode works (see updates to the README)
• Caveat 1: this probably breaks pre-gymnasium code (haven't tested it)
• Caveat 2: 'raw' rendering mode no longer supported.

[rhaps0dy]

You need to change from gym.envs.registration import register to from gymnasium.envs.registration import register in __init__.py.

[SimonOuellette35]

You need to change from gym.envs.registration import register to from gymnasium.envs.registration import register in __init__.py.

Done!