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

assets/settings/default.json

@@ -1350,6 +1350,8 @@
   //      "load_direnv": "direct"
   // 2. Load direnv configuration through the shell hook, works for POSIX shells and fish.
   //      "load_direnv": "shell_hook"
+  // 3. Don't load direnv configuration at all.
+  //      "load_direnv": "disabled"
   "load_direnv": "direct",
   "edit_predictions": {
     // A list of globs representing files that edit predictions should be disabled for.