UTF-8 windows code-page support

[davidrg]

Windows 10 version 1903 added a UTF-8 code page allowing the ANSI functions (eg, CreateFileA) to support UTF-8 strings. This is documented here:
https://learn.microsoft.com/en-us/windows/apps/design/globalizing/use-utf8-code-page

If K95 can be made compatible with this it will probably be a vastly easier approach to getting full unicode compatibility than implementing #142.

As the documentation notes, this doesn't affect GDI but calling the -W API variants for that is easy enough in any places we aren't already doing that.

Note that this really only affects things like filenames - K95 has supported UTF-8 terminal emulation for over 20 years