koi8ru and koi8t file encodings are not supported on Windows

[Colengms]

VS Code uses iconv for translating file encodings. (Actually https://github.com/microsoft/vscode-iconv-lite-umd ).

Files provided to extensions by VS Code are first converted to UTF-8. But,. for files that cpptools needs to open directly from disk (unopen header files), it needs to deal with encodings directly (i.e. to render doc comments correctly).

Cpptools uses iconv on Linux and Mac, but on Windows it currently uses Win32 APIs. There are 2 encodings explicitly supported by VS Code that are not supported by Win32 APIs:

• koi8ru - The nearest equivalent is 20866 (koi8-r), but is missing Belarusian.
• koi8-t - There is no equivalent code page, so no support available on Windows.

This could be addressed by using iconv on Windows as well.