made birch, spruce, fir, and jungle trees spawn again

turned out to be I was using foo:bar for their function calls, which
biome_lib filters out (only group defs can use that format).  Changed
them to quoted-string "foo.bar" format.  Also, jungle trees need a
moretrees-specific copy of the default jungle trunk node so that there's
something unique to match against for the density limit.

also, made jungle trees use the same biome settings as giant tree ferns

tree_models.lua

@@ -217,7 +217,7 @@ moretrees.jungletree_model={
 	axiom=nil,
 	rules_a=nil,
 	rules_b=nil,
-	trunk="default:jungletree",
+	trunk="moretrees:jungletree_trunk",
 	leaves="default:jungleleaves",
 	leaves2=nil,
 	leaves2_chance=nil,