Tower Intellect Module

[ZakShearman]

Add a module that players can purchase that allows them to choose what enemies a tower targets:

• [x] First
• [x] Weakest
• [x] Strongest
• [x] Closest
• [x] Furthest

Others:

• [ ] Implement these into a player-configurable module.

Also prevents the tower from targeting enemies that are invulnerable to their damage.

[ZakShearman]

Although all the targeting types are implemented for this, we'll keep it for later so the player-visible elements won't exist and the targeting cannot be changed.