Add a new shellmap

[MustaphaTR]

The blank snow shellmap really hurts my eyes. Simply changing it to temperate should be good enough for that issue but a proper one could be made too.

RV has shellmaps of 1st Training Mission and 7th Soviet Mission for a while now, but i think making another one special for OpenRA/RA2 would be better.

[abcdefg30]

I'd like to keep an empty one since I don't think the extra lag is justified during the testing phase of this mod. We could also just do it like TD. (Note that you can still file a shellmap, I just wouldn't set it in the shellmap category until we want to enable it for a release.)