Add Tree armor type and remove tree-only warheads

This simplifies #12467.
Using a tree-exclusive amor type is far more efficient
than adding more warheads, which cost performance
due to their huge Spread

This also restores the 100% efficiency vs. trees for
some of the incendiary nuke warheads (which have
reduced efficiency vs. Wood since #13643).

Note: Atomic had two tree-only warheads with a Delay
of 15, I believe the first one to be a copy-paste error
and moved the damage to the regular SpreadDamage
with a Delay of 10.

mods/ra/rules/defaults.yaml

@@ -898,7 +898,7 @@
 	Health:
 		HP: 50000
 	Armor:
-		Type: Wood
+		Type: Tree
 	Targetable:
 		TargetTypes: Trees
 	WithDamageOverlay@SmallBurn: