[MuJoCo] Add support for relative paths with xml_file arguments

[Kallinteris-Andreas]

Description

Adds support for using "." and "~" relative paths for the xml_file argument in mujoco-v3+ environments.
Previously one had to only full paths "/home/..."

Manual testing

>>> import gymnasium
>>> env = gymnasium.make('Humanoid-v5', xml_file="~/op3-scene.xml")
>>> env = gymnasium.make('Humanoid-v5', xml_file="./op3-scene.xml")

I have also added

def test_xml_file():
    """Verify that the loading of a custom XML file works"""
    env = PointEnv(xml_file="./tests/envs/mujoco/assets/walker2d_v5_uneven_feet.xml")
    assert env.unwrapped.data.qpos.size == 9

Fixes # (issue)

Type of change

• 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

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• [DNA] I have commented my code, particularly in hard-to-understand areas
• [DNA] 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