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`.

helix-stdx/Cargo.toml

@@ -15,6 +15,7 @@ homepage.workspace = true
 dunce = "1.0"
 etcetera = "0.8"
 ropey = { version = "1.6.1", default-features = false }
+which = "6.0"
 
 [dev-dependencies]
 tempfile = "3.9"