Add packaging support for setting codepage to UTF-8 in manifest

See this discussion and its continuation for discussion of how apps on MS can set a field in their manifest to ensure they get UTF-8 as their system encoding.

We should make it easy for Qt-using applications to do this when packaging and actively encourage them to do so. The MS-side of it all is now allegedly stable (where it was unstable when we made, at Qt 6, the transition to enforcing UTF-8 on other platforms).