Re-export `which` from `helix-stdx::env`

We use `which::which` in many crates, so `which` was a separate
dependency across all of them. We can centralize `which` into the
stdx crate so it's easy for all crates to depend on it.

I also moved the rest of `helix-view/src/env.rs` into helix-stdx's
`env` module since it only contained a thin wrapper around `which`
and `std::env`.

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