feat(core): Add the ability to use main thread for processing.

[petejohanson]

Add the ability to use a preemptible, and possibly lower priority, main thread for all core ZMK event/work processing, to keep system work queue available to process timimg sensitive work.

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.