Add minetest.get_mapgen_object to API

doc/lua_api.txt

src/emerge.cpp

src/emerge.h

src/jthread/jthread.h

src/jthread/pthread/jthread.cpp

src/jthread/win32/jthread.cpp

src/mapgen.h

src/mapgen_v6.h

src/mapgen_v7.h

src/script/lua_api/l_env.cpp

src/script/lua_api/l_env.h

src/script/lua_api/l_item.cpp

src/script/lua_api/l_vmanip.cpp

src/script/lua_api/l_vmanip.h