Added method to put the backpack in standby mode

[flupes]

Proposal to add the "missing" .end() method to cleanly put the HT16K33 in standby mode.

I needed this for a very low power application where the display is off most of the time. Without setting the HT16K33 to standby mode, it consumes ~100uA (no LED on) extra. (reduction from 130uA to 30uA with a ATmega328P in power-down mode).

• This is a new method, so it should not be breaking anything if not called by the users.
• This is tested only on a 8x8 Bicolor Matrix, so I cannot say anything about other displays.
• A test program comparing the .end() versus just clearing the display is available at: https://github.com/flupes/chronodouche/blob/main/test/low_power_led_backpack/test_low_power_led_backpack.cpp
• Power measurements are shown at: https://www.youtube.com/watch?v=fxPFTtWQyig