Add /emergeblocks command and core.emerge_area() Lua API

builtin/common/misc_helpers.lua

builtin/game/chatcommands.lua

builtin/game/misc.lua

doc/lua_api.txt

src/emerge.cpp

src/emerge.h

src/map.cpp

src/script/lua_api/l_env.cpp

src/script/lua_api/l_env.h