AI engagement distance

[cxong]

AI should want to move into the best range when engaging. For example a flamer AI should want to close the distance before engaging.

[cxong]

After refactoring weapon/bullet types (#64), it may be possible to programmatically determine a weapon's effective range (might be trickier for grenades and explosives). This way we don't have to specify range for custom weapons.

The AI should only fire when the target is almost within range, and back off if the enemy gets too close relative to the weapon's range.

We may still want to override that logic for certain weapons though, for example for the shotgun, engaging at close range is still beneficial.

To test this AI, create a custom map with stationary AIs with different weapons equipped. When the player is out of range, the AI should not fire.