Desert shellmap Lua maintenance

After lessons learned from https://github.com/OpenRA/OpenRA/pull/6377. Essentially a followup of https://github.com/OpenRA/OpenRA/pull/6284 to make use of the new reinforcement global and time utils. As the ant random number dice does not work (for me) at all, I replaced it with a Halloween easter egg. I also brought back the chinook reinforcements from the original C# implementation.
This should be `IsHalloween` to match the other `bool Is*` properties.