Add Spring.DropWreck

[sprunk]

Sometimes you may want to keep a unit technically alive after being killed (e.g. for correct damage attribution for any ongoing attacks). Delaying death this way forces you to spawn a wreck manually. It isn't trivial to replicate what the engine does perfectly:

• poll LuaRules for gadget:AllowFeatureCreation.
• where exactly does the wreck spawn, midpos? Base pos?
• with wrecklevel > 1 you gotta traverse the feature death chain manually.
• gotta set the original unit type for rezzing while obeying rezzability as defined in the featuredef
• inherit a bunch of traits such as velocity, heading, (ally)team.
• set smoke (this one is straight up not doable because there is no interface for setting smoke, but that is outside the scope of this request).
• engine behaviour may change in the future, outdating your gadget.

The request is to add Spring.DropWreck(unitID, number? wreckLevel) -> featureID that behaves the same as if the unit was dropping the wreck as part of its death sequence, but without dying.