* Use clang-tidy `misc-include-cleaner` for IWYU `#include` cleanup
* Use `std::optional<size_t>` instead of `ssize_t`
* Rename some functions in linkdefs.hpp
* Fix header order
* Implement warning diagnostic flags for RGBFIX
* `-m/--mbc-type help` prints to stdout
* Support `-m list` as well as `-m help`
* Make invalid `rgbfix -l` characters a fatal error, like other invalid CLI arguments
* Refactor fix/main.cpp into multiple files
