Avoid Duplicated "Theme File Editor" menu

[Mamaduka]

Description

PR adds a conditional check before moving "Theme Editor" under the "Tools", to avoid duplicated sub-menu items in WP 5.9.

Closes #37568.

How has this been tested?

1. Running WP 5.9 beta 4
2. Go to the Dashboard and click on the "Tools" menu
3. It should only display single "Theme File Editor" sub-menu item.

Screenshots

Before	After

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[Mamaduka]

Thank you, @aristath!