Added list envs function (pretty print envs)

[kad99kev]

Description

Added a function that lists all the current environments supported/present, neatly.
Resolves #75

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

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Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[pseudo-rnd-thoughts]

Thanks for the PR, could you add a test, just that this function works. Maybe add a couple of testing environments with different namespaces, etc and check the output is equal to an expected values.

Could you change the registry to add registry in __init__.py such that

# old code
registry: dict[str, EnvSpec] = {}

# new code
registry.pprint = list_env_spec
registry.__str__ = partial(list_env_spec, 10)

Another comment, change the function name to registry_pretty_print

My hope is being able to call print(gymnasium.registry) to get the pretty print versions
Then you can call print(gymnasium.registry.pprint(10)) for selecting the pretty print options