Add minetest.swap_node

games/minimal/mods/default/init.lua

src/client.cpp

src/client.h

src/clientserver.h

src/environment.cpp

src/environment.h

src/map.cpp

src/map.h

src/script/lua_api/l_env.cpp

src/script/lua_api/l_env.h

src/server.cpp

src/server.h