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/settings/src/settings_content/project.rs

@@ -507,6 +507,8 @@ pub enum DirenvSettings {
     /// Load direnv configuration directly using `direnv export json`
     #[default]
     Direct,
+    /// Do not load direnv configuration
+    Disabled,
 }
 
 #[derive(