feat(underglow): Add battery and BLE status to underglow

[fliano]

Adds functionality to the RGB underglow system, allowing it to display battery status and BLE connection status. Battery status is shown on startup and when reaching critical levels. BLE status is shown on startup and when the connection state changes.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.

[nVitius]

Haven't looked to closely at this implementation, but wanted to share w/ you the one that I made some months ago:
#2353

Mine is based on MoErgo's implementation for the Glove80, with some enhancements to make it work on any board via config. It works by displaying the configured statuses when a keycode is sent.

Unfortunately, I've had a very hard time getting a review from the maintainers.

[github-actions]

This PR has been automatically marked as stale because it has not had activity in 10 months. It will be closed in 14 days if no further activity occurs. Feel free to give a status update or re-open when it has been rebased and is ready for review (again). Thanks!