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"

helix-stdx/src/env.rs

@@ -1,3 +1,5 @@
+pub use which::which;
+
 use std::{
     path::{Path, PathBuf},
     sync::RwLock,
@@ -30,6 +32,14 @@ pub fn set_current_working_dir(path: impl AsRef<Path>) -> std::io::Result<()> {
     Ok(())
 }
 
+pub fn env_var_is_set(env_var_name: &str) -> bool {
+    std::env::var_os(env_var_name).is_some()
+}
+
+pub fn binary_exists(binary_name: &str) -> bool {
+    which::which(binary_name).is_ok()
+}
+
 #[cfg(test)]
 mod tests {
     use super::{current_working_dir, set_current_working_dir};