Update `pyproject.toml`, enable `mlx-lm` requirement only on darwin, disable `vllm` requirement on darwin

[lapp0]

• Fixes Discord user Vibhath's issue with mlx-lm on linux.
• Fixes https://github.com/outlines-dev/outlines/issues/989

Problem

pip install -e .[test] fails under two addressed conditions:

• gnu/linux user attempts to install mlx-lm dependency
• darwin user attempts to install vllm dependency.

Solution

Update pyproject.toml, ensure vllm and mlx-lm are installed only on appropriate systems.