usb/usb-device-keyboard: Add key release.

[amogha1234]

Micropython 1.23 preview version is used with Raspberry pi pico w board.
Trying to send keys using usb-keyboard library. It was clicking the key, but not releasing it, similar to #873 . Resulting in continuous sending key.
Hence added release_all() function, which releases all keys. And it worked with Rpi pico w.

[dpgeorge]

You should be able to release all keys by calling keyboard.send_keys(()), ie pass an empty tuple (or empty list [] but an empty tuple is more efficient because it doesn't allocate memory).

[amogha1234]

You should be able to release all keys by calling keyboard.send_keys(()), ie pass an empty tuple (or empty list [] but an empty tuple is more efficient because it doesn't allocate memory).

Thank you,
I thought it is better to have a dedicated function to release keys as it is present in other keyboard libraries of circuit-python and Arduino, it will be useful for the users.