Bushes: Add saplings

Bush saplings grow in half the time of tree saplings.
Bushes do not need 'from sapling' schematic variants because only the
stem node is force-placed in the mapgen schematic, so only the sapling
is force-replaced.
Bush leaves drop saplings with a rarity that ensures at least 1 sapling
per bush on average.

mods/default/crafting.lua

@@ -966,6 +966,18 @@ minetest.register_craft({
 	burntime = 10,
 })
 
+minetest.register_craft({
+	type = "fuel",
+	recipe = "default:bush_sapling",
+	burntime = 6,
+})
+
+minetest.register_craft({
+	type = "fuel",
+	recipe = "default:acacia_bush_sapling",
+	burntime = 7,
+})
+
 minetest.register_craft({
 	type = "fuel",
 	recipe = "default:aspen_sapling",