Clarify support for non-Windows targets

[kennykerr]

The windows-rs project is focused on bringing the best of Windows to Rust developers. Various efforts to support non-Windows targets continue to make it too difficult to support Windows, which is the whole point of this project.

The position we're taking is to provide basic non-Windows compatibility for windows-core only to support COM and nothing more. Even this adds regrettable complexity to the windows-core crate, but this update at least attempts to make that complexity manageable by providing a simple way for us to partition what parts of windows-core is specific to Windows and what is cross-platform. Subsequent updates will limit more of the code that isn't required for COM support.

The test_linux crate covers what little support we offer for non-Windows targets. This is limited to windows-core and windows-result and is tested in linux.yml as part of each build.

Fixes: #3101
Fixes: #3083
Related: #3125

[ChrisDenton]

This sounds like a good balance. Having support tightly scoped to only the essentials on non-Windows platforms still allows other parties to build on top of that if they wish. Just in a way that doesn't negatively impact windows-rs support for Windows.