``scan`` transformations for brainpy `Variable` system

[chaoming0625]

Please:

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• Describe your goal, and if possible provide a code snippet with a motivating example.

Although there is brainpy.math.for_loop, it does not support the loop over the non-Variable instances. Instead, jax.lax.scan supports iteratively changing the value over inputs.

[chaoming0625]

Has been supported.