We currently have a variety of functions named Utf8ToUtf16, Utf16FromUtf8, StringToWideString in various places throughout the codebase. Now that FML can be used in the platform-specific desktop embedders, move string-encoding functionality to FML and use that.

Windows APIs use wchar_t-based strings and as a result, we'll need to continue to keep Windows-specific functions in fml/platform/win/wstring_conversion.h. This does, however let us reduce our use of _SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING to a single instance.

Related: #50053