Jit-env

This release fixes a number of stale issues and updates the typing annotations in core.

This includes;
- Addition of `ExtendObservation` wrapper to nest TimeStep fields inside the observation (useful for POMDPs)
- Update all calls to `jax.random.PRNGKey` to `jax.random.key`.
- Some typing updates in `core` to make use of TypeVars. This is still incomplete and tracked in 28.
- Update all `chex` tests to remove `ignore_nones` arguments.

This release adds a Gymnasium converter for both Environment and Spec-Space.

It also fixes a bug inside the Tree spec and unpack_spec function that was incorrectly flattening and unflattening the custom defined PyTrees.

This release adds a StopGradient wrapper for the Environment Step function. This is a generally useful utility for online reinforcement learning algorithms.

This version also fixes not being able to use `State`, `Action`, or `Observation` as TypeVars as they were supposed to. These are now explicility imported from `_core.py` rather than defining TypeAliases.

This release removes the `render` method as an abstractmethod, and moves the rendering responsibility to an external argument dependency.

This release allows mypy to type check this library when installed from PyPI.

Changes:
- Added `py.typed` inside `jit_env` and `setup.py`
- Extended `repr` in Environment and Wrapper and implemented more detailed `repr`s in `jit_env.wrappers.py`.
- Added tests to ensure `repr` and `str` work as expected.

This release [0.1.0](https://pypi.org/project/jit-env/0.1.0/) covers the entire library adequately with tests to ensure reliable behaviour.


Future Development

Some features related to third-party library compatibility may be added later, such as `gymnasium`, or maybe `brax` or `gymnax`. When these additional features are implemented and tested, this library version will be bumped to `1.0.0`.



From this point on, features will only be added in the form of Pull Requests to prevent very sudden API changes.