feat: add Sys trait for swapping out system

[dsherret]

Creates a Sys trait and puts any system related functionality behind it.

This has several benefits:

1. Enables in-memory testing of this crate in the future (for now, I've only implemented a default RealSys implementation).
2. Allows consumers of this crate to not depend on env_home, rustix, and winsafe if they already have code or other dependencies for doing this.
   • Allows changing the behaviour of tilde expansion (for example, previously this crate relied on using winapi to get the home directory on Windows, but now it uses env_home, which only looks at the environment variables)
3. Opens the possibility to using this crate in wasm32-unknown-unknown as someone can provide their own which::Sys implementation.

[Xaeroxe]

I like this but can we adjust the presentation a bit? I'd consider this a feature for advanced users. So we should move the README example into the generated rustdocs. Additionally let's not flatten the sys module in our exports, just make the module public.

[Xaeroxe]

Also if you haven't already you should use your new trait in a proof of concept implementation for your own use case. It'd be a shame if we needed to revise the trait after this was already published.

[dsherret]

Also if you haven't already you should use your new trait in a proof of concept implementation for your own use case. It'd be a shame if we needed to revise the trait after this was already published.

Yes, that would be good. I'll look into doing that later. Going to convert this PR to draft until I find the time.