Add WASI feature

[Timmmm]

Opt in to feature(wasip2) when we're building for the wasip2 target. This feels a little redundant but the compiler demands it.

Not tested yet.

[Timmmm]

See rust-lang/rust#130323 (unfortunately this is not documented as far as I can see).

Actually based on the PRs linked to that issue maybe it should be

#![cfg_attr(
    all(feature = "nightly", target_os = "wasi", target_env = "p2"),
    feature(wasip2)
)]

but I'll do that when I get a response.

[dylni]

Thanks! I'll get this released soon.

I do not find gating on "nightly" necessary, but I appreciate you raising it. Anyone using this target with this crate will need to be on the nightly channel currently, so the feature will only be an extra step for them.