Let the TheaterCommander manage front line stance.

This improves the AI behavior by choosing the stances non-randomly:

* Breakthrough will be used if the base is expected to be capturable and
  the coalition outnumbers the enemy by 20%.
* Elimination will be used if the coalition has at least as many units
  as the enemy.
* Defensive will be used if the coalition has at least half as many
  units as the enemy.
* Retreat will be used if the coalition is significantly outnumbers.

This also exposes the option to the player.

game/commander/tasks/primitive/eliminationattack.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+from game.commander.tasks.frontlinestancetask import FrontLineStanceTask
+from gen.ground_forces.combat_stance import CombatStance
+
+
+class EliminationAttack(FrontLineStanceTask):
+    @property
+    def stance(self) -> CombatStance:
+        return CombatStance.ELIMINATION
+
+    @property
+    def have_sufficient_front_line_advantage(self) -> bool:
+        return self.ground_force_balance >= 1.0