feat: MUJOCO_LIB

[kanarus]

• close add MUJOCO_LIB path #44
• Along with [BREAKING] refactor: vendor MuJoCo's include directory in rusty_mujoco #45 that vendored MuJoCo's header files within rusty_mujoco and changed license, this PR is a breaking change that removes support for the MUJOCO_DIR environment variable.
• Introduces MUJOCO_LIB environment variable: Path of the directory containing libmujoco.so or mujoco.lib. (this is enabled by [BREAKING] refactor: vendor MuJoCo's include directory in rusty_mujoco #45)
• At the same time, improves library-probing approach to handle system-wide mujoco library without MUJOCO_LIB or any other environment variable, and to assert that the found mujoco library can be successfully linked to an executable.
• Not tested enough on Windows and Mac yet.