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-loader/src/grammar.rs

@@ -86,7 +86,7 @@ pub fn get_language(name: &str) -> Result<Language> {
 }
 
 fn ensure_git_is_available() -> Result<()> {
-    match which::which("git") {
+    match helix_stdx::env::which("git") {
         Ok(_cmd) => Ok(()),
         Err(err) => Err(anyhow::anyhow!("'git' could not be found ({err})")),
     }