win32: request the UTF-8 code page for Windows APIs

This sets the activeCodePage property in the manifest, which forces the
ANSI code page to be UTF-8 in Windows 10 1903 and up. It shouldn't make
a difference for mpv itself, since mpv already uses the wide-char APIs
for most functions, however some of mpv's dependencies, such as Lua,
rely on the ANSI codepage. Hence this change enables support for Unicode
file names in Lua's I/O library.

Thanks @avih for finding this property.

See:
https://docs.microsoft.com/en-us/windows/uwp/design/globalizing/use-utf8-code-page

osdep/mpv.exe.manifest

@@ -11,6 +11,7 @@
         <windowsSettings>
             <dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">True/PM</dpiAware>
             <dpiAwareness xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2,PerMonitor</dpiAwareness>
+            <activeCodePage xmlns="http://schemas.microsoft.com/SMI/2019/WindowsSettings">UTF-8</activeCodePage>
         </windowsSettings>
     </application>
     <trustInfo xmlns="urn:schemas-microsoft-com:asm.v2">