Add exhaustion tracking

[mpirnat]

It doesn't happen often, but when it does, it would be nice to track the exhaustion level of a combatant, because the consequences get serious quickly.

We should:

• add a new attribute on Combatant to track exhaustion; default to 0. Min 0, max 6.
• emit the exhaustion level with the conditions when getting details on a combatant
• emit the exhaustion level with the conditions when showing the party or showing the monsters in the terminal interface
• emit the exhaustion level with the conditions when refreshing JSON data for the player view
• add a command to modify the exhaustion of a combatant

The command interface could look like:

> exhaustion Dewain
Dewain has 0 points of exhaustion.
> exhaustion Dewain 3
Dewain has 3 points of exhaustion.
> exhaustion Dewain +1
Dewain has 4 points of exhaustion.
> exhaustion Dewain Lander -1
Dewain has 3 points of exhaustion.
Lander has 1 point of exhaustion.

Exhaustion and its effects are described here: https://www.dndbeyond.com/sources/phb/appendix-a-conditions#Exhaustion