Add offhand support to lib trees (#1223)

Author: runcows

lib_trees/data/gm4_trees/function/sapling/summon_marker.mcfunction

@@ -4,10 +4,16 @@
 # run from gm4_trees:sapling/place_sapling
 
 # set up storage to be read from
+# mainhand
 data modify storage gm4_trees:temp sapling.type set from entity @s SelectedItem.components."minecraft:custom_data".gm4_trees.item.type
 ## backwards compatibility with old items
 execute unless data storage gm4_trees:temp sapling run data modify storage gm4_trees:temp sapling.type set from entity @s SelectedItem.components."minecraft:custom_data".gm4_fruiting_trees.item.type
 
+# offhand
+execute unless data storage gm4_trees:temp sapling run data modify storage gm4_trees:temp sapling.type set from entity @s equipment.offhand.components."minecraft:custom_data".gm4_trees.item.type
+## backwards compatibility with old items
+execute unless data storage gm4_trees:temp sapling run data modify storage gm4_trees:temp sapling.type set from entity @s equipment.offhand.components."minecraft:custom_data".gm4_fruiting_trees.item.type
+
 # call initialize function so other modules can add their markers
 # | NOTE positioned ~.5 ~.5 ~.5 to not break data packs that relied on this behavior of old versions of this library
 execute positioned ~.5 ~.5 ~.5 run function #gm4_trees:initialize_sapling