Re-implement Shadow support for Missiles

Will be needed for TS, at least.

Also took this opportunity to remove lots of formerly bitrotted "Shadow: no/false" entries from the shipping mods. Shadow defaults to false as that's the wanted behaviour for all currently shipping mods, so these entries are obsolete anyway.

I intentionally left TS' weapons.yaml as-is, as a constant reminder that we need something like #3680 to make missile flight behaviour look good & realistic for TS/RA2 or similar projects which use isometric perspective.
You can only substract WVec from pos, but I guess this wants to be

`var shadowPos = new WPos(pos.x, pos.y, 0);`