Add try_exists function as wrapper over std::path::Path::try_exists

[kanpov]

No description provided.

[kanpov]

@notgull ping

[kanpov]

We can replicate behavior of the MSRV-incompatible std::fs::exists manually, but this approach is a simpler one that just uses std::path::Path::try_exists instead. This is what Tokio's tokio::fs::try_exists does, so I'd say it'd be a reasonable expectation for async_fs to mirror that functionality.

[taiki-e]

Having try_* without the non-try_ variant sounds odd.

See also the stabilization report of std::fs::exists in rust-lang/rust#83186 (comment).