unit: Add SimpleDualButtonUnit for callback-free polling

[amin45mh]

Add SimpleButton class and SimpleDualButtonUnit factory that wrap machine.Pin directly, providing simple polling methods (value, is_active, was_pressed, was_released) without requiring tick() or callbacks. The existing DualButtonUnit is unchanged.

[hlym123]

Thanks for the contribution. We adopted this idea and merged a revised
implementation into the develop branch in f44294d.

The merged version keeps the existing DualButtonUnit API unchanged and adds
SimpleButton and SimpleDualButtonUnit for callback-free polling. value() and
is_active() read machine.Pin directly, while update() is only required once
per loop for debounced was_pressed() and was_released() edge detection.

The revised implementation also adds configurable debounce_ms, correct
pull-up/pull-down handling for active_low, initialization from the current
input level, a MicroPython example, and English/Chinese documentation.

Since this functionality is now available in develop, we will close this PR.
Thanks again for the contribution.