Workbench: Make custom cuttable nodes table global

src/workbench.lua

 local workbench = {}
+WB = {}
 screwdriver = screwdriver or {}
 local min, ceil = math.min, math.ceil
 
@@ -28,7 +29,7 @@ for node, def in pairs(minetest.registered_nodes) do
 end
 
 -- Optionally, you can register custom cuttable nodes in the workbench.
-workbench.custom_nodes_register = {
+WB.custom_nodes_register = {
 	-- "default:leaves",
 }
 
@@ -41,7 +42,7 @@ setmetatable(nodes, {
 	end
 })
 
-nodes = nodes..workbench.custom_nodes_register
+nodes = nodes..WB.custom_nodes_register
 
 -- Nodeboxes definitions.
 workbench.defs = {