refactor(underglow): Proper settings commit hook, startup/stop fixes

[petejohanson]

Use a settings handler commit hook to only start RBG if the most recent loaded saved stated has underglow enabled. Additionally, refactor our off code to use a timer stop hook instead of directly submiting the work item to stop the underglow.

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.