direnv: Allow disabling env integration entirely (#43764)

Relates to #35759, but maybe doesn't entirely fix it? I think it will
improve the situation, at least.
Also provides a workaround for the issue described in
https://github.com/zed-industries/zed/issues/40094#issuecomment-3559808526
for users of WSL + `nix-direnv`.

Rationale: there are cases where automatic direnv integration is not
always desirable, but Zed currently has no way of opting out of this
integration besides `direnv revoke` (which is often not desirable).

This PR provides such an opt-out for users who run into problems with
the existing direnv integration methods. Some reasons why disabling
might be useful:
- Security concerns about auto-loading `.envrc` (arguably, `direnv
revoke` should cover this most of the time)
- As in #35759, for users who use different shells/envs for
interactive/non-interactive cases and want to manually control the
environment Zed uses
- As in #40094, to workaround OS limits on environment variable /
command-line parameter size


Release Notes:

- Added the ability to disable direnv integration entirely

crates/project/src/environment.rs

@@ -314,6 +314,10 @@ async fn load_directory_shell_environment(
     load_direnv: DirenvSettings,
     tx: mpsc::UnboundedSender<String>,
 ) -> anyhow::Result<HashMap<String, String>> {
+    if let DirenvSettings::Disabled = load_direnv {
+        return Ok(HashMap::default());
+    }
+
     let meta = smol::fs::metadata(&abs_path).await.with_context(|| {
         tx.unbounded_send(format!("Failed to open {}", abs_path.display()))
             .ok();
@@ -355,6 +359,7 @@ async fn load_directory_shell_environment(
     // even if direnv direct mode is enabled.
     let direnv_environment = match load_direnv {
         DirenvSettings::ShellHook => None,
+        DirenvSettings::Disabled => bail!("direnv integration is disabled"),
         // Note: direnv is not available on Windows, so we skip direnv processing
         // and just return the shell environment
         DirenvSettings::Direct if cfg!(target_os = "windows") => None,