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

docs/src/configuring-zed.md

@@ -584,10 +584,11 @@ Note: Dirty files (files with unsaved changes) will not be automatically closed
 
 **Options**
 
-There are two options to choose from:
+There are three options to choose from:
 
 1. `shell_hook`: Use the shell hook to load direnv. This relies on direnv to activate upon entering the directory. Supports POSIX shells and fish.
 2. `direct`: Use `direnv export json` to load direnv. This will load direnv directly without relying on the shell hook and might cause some inconsistencies. This allows direnv to work with any shell.
+3. `disabled`: No shell environment will be loaded automatically; direnv must be invoked manually (e.g. with `direnv exec`) to be used.
 
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