Create base AoE damage/heal classes

[shawncplus]

Related to #60

Use-case

• Player A uses a skill which coats the room in oil (adding an Oil-Covered effect to the room).
• Player B uses an area of effect fire spell. The room's oil effect should be able to listen for AoE fire damage and explode dealing extra damage

Solution

Create new AreaOfEffectDamage and AreaOfEffectHeal classes in core which extend Damage and Heal respectively. They will have two differences from base damage/heal:

• A Character[] getValidTargets(Room|Character commitTarget) method which will find valid targets based on where the damage is committed, e.g., if the commit target is a room it will return all players/npcs in the room. This method can be overridden by a subclass to customize valid targets.
• An override of the commit(Character target) method to accept Room|Character target. If the target is a Room then it should call getValidTargets(room), call super.commit(target) for each valid target, then finally emit a areaDamage or areaHeal event on the room passing in AreaOfEffectDamage/Heal instance and the commit target.

class AreaOfEffectDamage extends Damage
{
  /**
   * @param {Room|Character} target
   */
  commit(room) {
    if (!(room instanceof Room)) {
      super.commit(target);
      return;
    }

    const targets = this.getValidTargets(room);
    for (const target of targets) {
      super.commit(target);
    }

    room.emit('areaDamage', this, targets);
  }

  /**
   * Override this method to customize valid targets such as
   * only targeting hostile npcs, or only targeting players, etc.
   * @param {Room} room
   * @return {Array<Character>}
   */
  getValidTargets(room) {
    return [...room.npcs];
  },
}