Allow setting multiple values for ArtilleryFlarePrototype::shot_category

[JohnTheCF]

Currently, a flare placed by an artillery remote can either have no filtering for an ammo category or restrict to one category. It would be useful to be able to select multiple ammo categories for the flare.

My mod has a separate ammo category for a Plutonium artillery shell. It can be used in regular artillery turrets (which was requested and added in viewtopic.php?p=468553), but gets different damage bonuses to mirror the Uranium rocket nuke. In vanilla, the artillery targeting remote's flare doesn't have any filtering and the Plutonium nuke works just fine, but when some other mod changes that, manual targeting breaks. I've got that issue reported to me today on mod portal: https://mods.factorio.com/mod/Plutonium ... bb13741875

I've temporarily fixed the manual targeting by making the Plutonium nuke use the same ammo category as the regular artillery shell and doubling the damage to compensate for having less damage bonuses compared to Uranium nuke.

So, not limiting the flare's filtering to just one ammo category would be very helpful in this case.