ignore: add support for WASI

[Timmmm]

Add support for compiling to wasm32-wasip2. For this target WalkParallel becomes single threaded. This allows projects that depend on ignore to be compiled for WASI without having to modify their code.

Note this depends on a corresponding PR for walkdir so it won't compile currently.

To test add this to Cargo.toml:

[patch.crates-io]
walkdir = { git = "https://github.com/Timmmm/walkdir.git", branch = "user/timh/wasi" }

[BurntSushi]

See conversation in BurntSushi/walkdir#205

In summary, unless there is an exceptionally good reason to, I generally don't add support for nightly only stuff to libraries. I've done it in the past, and it's almost always been a maintenance headache.

I'd be happy to have these PRs re-submitted once they use stable APIs.