Allow PWM control of display brightness

`peripherals.brightness` (`board.BACKLIGHT` on the PCA9554 I/O expander) is enabled as an output on startup, and is set originally to `True`. Setting it to False leaves it as an output.

If the A1/PWM jumper is soldered closed, the code above prevents PWM output on `board.A1` from adjusting the brightness, because the output current can override the PWM output.

If board.BACKLIGHT` on the PCA9554 is set to an input, then it has two weaker pullups on it: one is a 10k resistor on the board, and the other is the built-in pullup on the PCA9554.

This can be done in a hacky way by doing `peripherals._backlight.switch_to_input(pull=Pull.UP)`. But it would be better to have it in the API.

How about allowing `peripherals.backlight = None`? This would do a `switch_to_input()` as above. If the jumper is solder closed, then this has been shown to allow PWM-ing the brightness. The API documentation should explain this and explain that the jumper must be soldered closed.

There _could_ also be an API added to do the PWM-ing.

See https://discord.com/channels/327254708534116352/537365702651150357/1439017635697262735 and below (and a little above).

Start by tracing the peripherals.backlight API and its board.BACKLIGHT handling on the PCA9554. Confirm whether the intended scope is accepting None, adding PWM control, or both; done should include the selected behavior and API documentation explaining the A1/PWM jumper requirement.

Written by the indexing model from the issue text.