Add core.spawn_tree_on_vmanip (#15415)

This function works like `core.spawn_tree`, but spawns an L-system tree onto a VoxelManip object instead on the map.

doc/lua_api.md

@@ -6588,6 +6588,9 @@ Environment access
       on-the-fly
 * `core.spawn_tree (pos, {treedef})`
     * spawns L-system tree at given `pos` with definition in `treedef` table
+* `core.spawn_tree_on_vmanip(vmanip, pos, treedef)`
+    * analogous to `core.spawn_tree`, but spawns a L-system tree onto the specified
+      VoxelManip object `vmanip` instead of the map.
 * `core.transforming_liquid_add(pos)`
     * add node to liquid flow update queue
 * `core.get_node_max_level(pos)`