RGB Underglow: Auto off on idle

[infused-kim]

This PR adds an auto off on idle feature to the underglow. It's already supported for the backlight and this adds a similar setting to the underglow.

I have tested it on my sofle choc and it's working great.

[infused-kim]

I realized my initial commit had a bug.

zmk_rgb_underglow_offand zmk_rgb_underglow_on save the state to the config. So if the user has underglow on, then idles and the underglow is auto-turned off, that setting saved into the flash. If the user turns off and on their keyboard, it starts in the underglow off state even though the user never changed that setting.

The new commit ensures the auto-off feature is only active if the underglow is on and that it doesn't save the temporary off state into the config.

[petejohanson]

Thanks for the PR, @infused-kim!

Can you please try with the latest main? You'll need to enable the new Kconfig option to enable the functionality.

[autoferrit]

This seems like a smaller change. is anything other than review needed to get this in? is there any way I can help? I am new to c/zmk but could try.

[lesshonor]

This PR appears to have been obsoleted by #1010, which was merged in June 2023.