Implement Initial Simple Attacking

[Indy2222]

• All entities has max HP and current HP.
• Entities with attacking capability have something like LaserGun { damage: f32, rate: f32, last_fire: Option<Time> }
• When an entity reached HP of 0, it is destroyed.
• There is a new EntityDestroyed event handled by all relevant plugins (to remove dangling Entity IDs).