Guard `windows::ffi::OsStringExt` usage behind `#[cfg(windows)]` for WSL and Linux support

[MarijnS95]

Motivation

We'd like to run - run, not cross-compile - DirectXShaderCompiler directly on Linux. At the same time, @riverar found a case to run dxcore inside WSL, which is also a Linux target.

The windows crate doesn't compile for / run on non-Windows targets currently, as std::os::windows::ffi::OsStrExt isn't available on target_os != "windows".

A proposed fix guards all trait implementations containing std::os::windows::ffi::OsStrExt behind a #[cfg(windows)] flag. If needed, followup PRs can introduce Linux-specific implementations for these conversion traits (but read on for a 32-bit wchar caveat).

Drawbacks

Extra complexity for a "purely Windows-oriented" crate. We can solve that by build-testing for Linux - in addition to cross-compiling from it.

Rationale and alternatives

Impact: Not supporting windows-rs on Linux and WSL while at the same time deprecating com-rs leaves users with no alternative for cross-platform COM bindings into libraries like DirectXShaderCompiler.

Additional context

Prior art: #1842 (comment)

Caveat

Furthermore, non-Windows targets use 32-bit widechars instead of 16-bit:

https://en.cppreference.com/w/cpp/language/types#Character%20types:~:text=wchar_t,a%20distinct%20type.

32 bits on systems that support Unicode. A notable exception is Windows, where wchar_t is 16 bits and holds UTF-16 code units

This should be addressed in a separate issue, and seems to be trivially implementable using Rust's char type. This is also how https://docs.rs/widestring implements it.

[riverar]

Yep, my approach was just to make those helpers unavailable to non-Windows targets. But if we can maintain those APIs and just rewrite the impls to work across platforms, even better. 👍

[MarijnS95]

@riverar Yeah I'd start out with just that, don't think I need it for my DXC wrapper anyway.

I would've let you do the honors to make a PR since it was your suggestion, but I had to make them myself to build-test the windows crate so I might as well submit what I have :)