minetest.register on hpchange

Description
Register a callback to be called when player's health is changed.

recently added https://github.com/minetest/minetest/commit/aa13baa30a45b0f834c23bd5c0407895eb8ec0ee

Arguments

 * 1) func(player, hp_change) - function that accept 2 arguments, called on change of health
 * 2) player - ObjectRef of player
 * 3) hp_change - the amount of change. Negative when it is damage.
 * 4) modifier - when true, the function should return the actual hp_change.