added custom groups to register_plant food craftitem

cotton.lua

@@ -146,5 +146,6 @@ minetest.register_node("farming:cotton_8", table.copy(crop_def))
 farming.register_plant("farming:cotton", {
 	description = "Cotton seed",
 	inventory_image = "farming_cotton_seed.png",
+	groups = {flammable = 2},
 	steps = 8,
 })]]

init.lua

@@ -513,6 +513,7 @@ farming.register_plant = function(name, def)
 	minetest.register_craftitem(":" .. mname .. ":" .. pname, {
 		description = pname:gsub("^%l", string.upper),
 		inventory_image = mname .. "_" .. pname .. ".png",
+		groups = def.groups or {flammable = 2},
 	})
 
 	-- Register growing steps