Allow opening single directory in Files panel with editor

[ivomac]

• PR Description

The openDirInEditor command is only used in the worktrees panel. It would make sense to use it in the Files panel if the user edits a directory by pressing e.

When a mix of directories and files are selected, the current behavior is to pass only the file paths to the editor, which is maintained. Editing only directories is currently disabled (ErrCannotEditDirectory). I have changed it so that it is possible to edit a single directory.

A new DisabledReason ErrCannotEditMultipleDirectories seems necessary.

Related Issue: #4512

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
  • I'm not sure where the tests that involve the editor are. A test that attempts to edit a single dir would make sense though.
• Text is internationalised (see here)
  • Not relevant yet. Would be needed for a new disabled reason.
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
  • New UserConfig not needed.
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc