Cache result with once_cell

[rgwood]

Hello! Thanks for creating this crate; I spotted it in my GitHub feed and was quite happy because the older wsl crate has been abandoned. Hopefully is-wsl can be a nice replacement for wsl.

If you're open to PRs, this PR caches the is_wsl() result with once_cell. It's similar to a PR I made for wsl, which was motivated by a situation in Nushell where we want to (frequently) check WSL status before rendering interactive content.

I know it's not ideal to add a dependency, but once_cell is extremely widely used, so much so that it's already in std on nightly. Once it's stabilized, the external dependency will no longer be needed.

I also added an example (cargo run --example main) just to make manual testing a bit easier.

[TheLarkInn]

Wow thank you so much for this! I'm going to find some time today to review this!

Adding a crate for caching is a great decision since we should save the work from the file system calls.

[TheLarkInn]

LGTM! Thank you for both the example and the optimization!

I'll create an issue for removing once_cell once it goes from nightly => stable in std