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-dap/src/client.rs

@@ -113,7 +113,7 @@ impl Client {
         id: usize,
     ) -> Result<(Self, UnboundedReceiver<Payload>)> {
         // Resolve path to the binary
-        let cmd = which::which(cmd).map_err(|err| anyhow::anyhow!(err))?;
+        let cmd = helix_stdx::env::which(cmd).map_err(|err| anyhow::anyhow!(err))?;
 
         let process = Command::new(cmd)
             .args(args)