Added despawn delay for dead enemies, rehauled AI Manager with spawn zones, and added difficulty scaling via LevelData

[skar32]

--- Dead enemies despawn 15 seconds after being killed --- AI Manager has been refitted with spawn zones; spawn points and respawn points are now obsolete

• New "Enemy Spawn Zone" prefab that contains box collider (dimensions can be modified as needed in editor)

--- Added two new variables to "LevelData" prefab: "enemyNumSpawns" and "enemyIncrementMaxAlive"

• "enemyNumSpawns" -> how many of this enemy type spawn on level start
• "enemyIncrementMaxAlive" -> how many more enemies of this type can be alive at once during each difficulty scaling pass

--- Added difficulty scaling that increases the max amount of enemies that can be alive at once

• Occurs once every 60 seconds for a maximum of 5 passes (5 minutes)

--- New AI Manager Logic: (1) Upon level start, AI Manager selects a random 3D transform within a random spawn zone, and spawns an enemy there if the transform does not intersect with another game object (2) AI Manager calculates the number of each enemy type that is alive, and if it is less than their corresponding max alive count, respawns instances of that enemy type until the number of alive instances is equal to their max alive count (3) AI Manager detects optimal spawn zones to use for enemy respawning; spawn zones that are more than 100 units away from the player and not currently visible are optimal

--- Best Practices:

• Place spawn zones slightly above the ground
• Make sure game objects that comprise the ground of a level are given the "Ground" layer mask
• Place an abundant number of small spawn zones that are scattered across the level