Callback that runs when the players health is updated

Something like MC_PLAYER_HEALTH_UPDATE.
While we have post damage callbacks, there’s a lot of other ways the players health can change. If we wanted to make a feature which did something when the players health changes, while not impossible and likely just needing to be ran/checked every frame, this would make it a heck of a lot easier/nicer, as it would be less code running every frame.
Ideally this would fire not just when the player gains/loses health, but also whenever they get a health up, health down, broken heart, full heart reroll, etc — basically, anything that changes the players hp.
This would have no return parameters as it is merely for checking the values, not changing them.

Start by reviewing the existing post-damage callback implementation and tracing the player-health changes described in the issue, including health ups, health downs, broken hearts, and full-heart rerolls. Done means a callback fires for each relevant health change, exposes no return parameters, and avoids requiring a per-frame check.