This makes the distinction between an _environment_ and an _experiment_ clearer.

If users want to import individual environments for their own debugging/development:

✗ from bsuite.experiments.catch import catch
✓ from bsuite.environments import catch

This change also introduces some more formal typing of bsuite environments:
- Add a base class which includes the bsuite_* attributes/methods.

PiperOrigin-RevId: 307575828
Change-Id: Iba2303d64a397ccef8a3f3f154e414bf343f905b