Corresponding to official release version 1.18.
Added VS2026 projects and solutions.
Known issue: MT DLL build is questionably tricky to use correctly that iconv relies on returning error via errno and when MT runtime is used in libiconv.dll, the instance of errno that libiconv.dll uses would be different from the instance of errno that client code sees. Not fixing that in this release and this is not new problem specific to the newer libiconv version. Just something has never been tested before and overlooked.