Convert existing Gym spec to Gymnasium

[jjshoots]

This is a pretty brutal PR, what it does is:

1. Drops gym support.
2. Removes the use of 4 different locations for ROMs, that's not really needed anymore since all ROMs will be stored within the Python package on PyPI in future versions.
3. Moves the ROMs directly into the roms folder.
4. Streamlines the Python files to be less scattered.

To-do:

• Fix tests, I'm not sure what the exact way to do this is and it depends on what we want to support
• Better pre-commits