minetest.find nodes in area under air

Description

 * returns a list of positions
 * returned positions are nodes with a node air above
 * `nodenames`: e.g. `{"ignore", "group:tree"}` or `"default:dirt"`
 * faster than using minetest.find_nodes_in_area and searching for air
 * Area volume is limited to 4,096,000 nodes
 * available since 0.4.12-dev