Add callback parameter for core.emerge_area()

builtin/game/chatcommands.lua

builtin/game/constants.lua

builtin/game/init.lua

doc/lua_api.txt

src/script/cpp_api/s_env.cpp

src/script/cpp_api/s_env.h

src/script/lua_api/l_env.cpp

src/script/lua_api/l_env.h

src/server.h