Add support for temporary hit points

[mpirnat]

Having temporary hit points is surprisingly more common than I expected, so we should finally add support for it.

Temporary hit points are explained here: https://www.dndbeyond.com/sources/basic-rules/combat#TemporaryHitPoints

We should add a temp_hp attribute to combatants. It can default to 0, with a minimum of 0.

The cur_hp getter should return the sum of _cur_hp and temp_hp.

The cur_hp setter should be modified to check if the new value is less than _cur_hp, and if so, deduct as much of the difference as it can from temp_hp, with the remainder from difference - temp_hp applied to the _cur_hp. If we get this right here, it means that we don't have to change any of the code that adjusts cur_hp!

temp_hp should be emitted into the toml when saving player state, and should be reloaded when loading the party.

We can optionally add a temp_hp command to set the temp_hp on a combatant, or we can lean on the existing alter command which would already be able to do alter <combatant> temp_hp <value>.

[mpirnat]

I punted on writing a new command for setting temp_hp for now since I can just do alter <combatant> temp_hp <value> and it works fine as long as the value provided is a non-negative integer.