fix: support wasm32-wasip2 on the stable channel

[brooksmtownsend]

Hey, me again 👋🏻 😅

As of today, Rust 1.82.0, the wasm32-wasip2 target is now in the stable channel. My previous PR #960 only accounted for the nightly channel, since the feature(...) directive is enabled there. Unfortunately it meant that my changes didn't allow for using this crate in a project on the stable channel.

After looking at the feedback in the last PR, and the comments following in rust-lang/rust#130323, I thought it might be useful to try and fix this. So in this PR I:

1. Removed the wasm32-wasip2 unstable feature gate
2. Replaced the use of std::os::wasi::prelude::OsStrExt with a to_string_lossy conversion, bearing in mind that as per wasip2 target should not conditionally feature gate stdlib APIs rust-lang/rust#130323 this is practically infallible since WASI paths are valid UTF-8
3. Replaced the use of std::os::wasi::prelude::OsStrExt with a String::from_utf8(bytes).map(|path| PathBuf::from(path)) conversion, bearing in mind that as per wasip2 target should not conditionally feature gate stdlib APIs rust-lang/rust#130323 this is practically infallible since WASI paths are valid UTF-8. In the case that this would fail, I return an error instead to follow the convention of that function.

I tried to fit existing code patterns as best I could, but I'm open to changing any of the code or even dropping back to an issue to discuss. There are some really interesting projects like reqwest & waki that are starting to get some wasm32-wasip2 support, which is why I'm making these PRs 🙂

[codecov]

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