Fix: don't stop seperation of MIRV warheads after launching player died (#3134)

## Description:
Like Atom and H-bombs, MIRV warheads should still land after the
launching player died. PlayerExecution already makes sure to skip nukes
when deleting a dead player's units. But MIRVexecution separate()
creates NukeExecutions which check canBuild, which returns false for a
dead player.

Fix: Skip alive and cost check for MIRV warheads. canBuild is only
applicable when creating the MIRV itself, not at the seperation stage
mid-flight. There's no cost involved either.

Note: this bug has been there since MIRVs were added to the game. But
the bug wasn't noticed often in all that time. Still the fix may have
some impact on gameplay.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

tryout33

Co-authored-by: Ryan <7389646+ryanbarlow97@users.noreply.github.com>

src/core/game/PlayerImpl.ts

@@ -988,7 +988,10 @@ export class PlayerImpl implements Player {
     }
 
     const cost = this.mg.unitInfo(unitType).cost(this.mg, this);
-    if (!this.isAlive() || this.gold() < cost) {
+    if (
+      unitType !== UnitType.MIRVWarhead &&
+      (!this.isAlive() || this.gold() < cost)
+    ) {
       return false;
     }
     switch (unitType) {