Unit cap (#4689)

* Add unit caps as a map setting.

* Use int and add maximum and minimum handling.

* Put a lower limit of 0 for unit cap

core/src/mindustry/game/Rules.java

@@ -44,6 +44,8 @@ public class Rules{
     public boolean fire = true;
     /** Whether units use and require ammo. */
     public boolean unitAmmo = false;
+    /** Whether cores add to unit limit */
+    public boolean unitCapVariable = true;
     /** How fast unit pads build units. */
     public float unitBuildSpeedMultiplier = 1f;
     /** How much damage any other units deal. */