Add an MSIX packaging option for Windows

[freakboy3742]

What is the problem or limitation you are having?

Windows appears to have added a new app distribution format - MSIX. This format appears to be preferred for some usages, so it should be supported by Briefcase.

Describe the solution you'd like

briefcase package windows -p msix should be a valid packaging command.

Describe alternatives you've considered

Do nothing, continue to support MSI and ZIP packaging.

Additional context

Our existing WIX tooling needs to be upgraded (see #1185), but WiX documentation is... not great.

The documentation of MSIX is... dense, and doesn't spend a lot of time explaining how MSIX fits into the existing MSI ecosystem. However, from an initial read, it appears that MSIX might (maybe?) be compatible with Windows App Store distribution.

Another advantage of MSIX appears to be that Microsoft itself is providing the tooling to build the installers, so we don't need to rely on a badly documented third-party tool to meet Windows packaging needs.

The first step to progress this will be to investigate the new format and tools and see what is involved in turning a "directory of content" into a signed MSIX installer.

[freakboy3742]

This draft PEP discussing the Windows installer landscape for CPython gives some background that might be helpful in untangling the installer landscape. In particular, it seems to suggest that MSI is effectively a "dead end", and the focus is now on app-store distribution (using MSIX, which is apparently also referred to as APPX).