feat(split): add peripheral changed event on central

[ReFil]

Add a new event for when any peripheral connects or disconnects from the central

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.

[petejohanson]

Thanks for working on this! A few thoughts.

[petejohanson]

Honestly, it'd be clearer if using just conditional branches for both, IMHO:

Suggested change

	if (CONFIG_ZMK_SPLIT AND (NOT CONFIG_ZMK_SPLIT_ROLE_CENTRAL))
	target_sources(app PRIVATE src/events/split_peripheral_status_changed.c)
	endif()
	target_sources_ifdef(CONFIG_ZMK_SPLIT_ROLE_CENTRAL app PRIVATE src/events/split_central_peripheral_status_changed.c)
	if (CONFIG_ZMK_SPLIT)
	if(CONFIG_ZMK_SPLIT_ROLE_CENTRAL)
	target_sources(app PRIVATE src/events/split_central_peripheral_status_changed.c)
	else()
	target_sources(app PRIVATE src/events/split_peripheral_status_changed.c)
	endif()
	endif()