Add reaction JSON models

CMakeLists.txt

include/mtx/events.hpp

include/mtx/events/collections.hpp

include/mtx/events/common.hpp

include/mtx/events/encrypted.hpp

include/mtx/events/messages/audio.hpp

include/mtx/events/messages/emote.hpp

include/mtx/events/messages/file.hpp

include/mtx/events/messages/image.hpp

include/mtx/events/messages/notice.hpp

include/mtx/events/messages/text.hpp

include/mtx/events/messages/video.hpp

include/mtx/events/reaction.hpp

lib/structs/events.cpp

lib/structs/events/collections.cpp

lib/structs/events/common.cpp

lib/structs/events/encrypted.cpp

lib/structs/events/messages/audio.cpp

lib/structs/events/messages/emote.cpp

lib/structs/events/messages/file.cpp