[feat] Support brainunit.Quantity type value param in creating Variable

[Routhleck]

This pull request introduces several changes to the brainpy project, focusing on enhancing the Variable class and updating test cases. The most important changes include the addition of new dependencies, modifications to the Variable class to support a new state_mode, and updates to the test cases to reflect these changes.

Enhancements to Variable class:

• brainpy/_src/math/object_transform/variables.py: Modified the Variable class to inherit from State and added a state_mode parameter to the constructor. This allows the Variable class to handle Quantity types appropriately. [1] [2] [3]

Updates to test cases:

• brainpy/_src/math/object_transform/tests/test_variable.py: Updated the test cases to import brainunit and jax.numpy, and added a new test_state method to test the new state_mode functionality of the Variable class. [1] [2]

Dependency updates:

• requirements-dev.txt: Added brainunit to the development requirements.
• setup.py: Added brainstate and brainunit to the installation requirements.