Add check for top-level ee directory in CE repo

What does this MR do?

Adds a simple script that checks if ee directory is present in the repository. Add this check to CI so build fails if someone commits ee directory in CE repo.

Are there points in the code the reviewer needs to double check?

no

Why was this MR needed?

To prevent accidentally commit ee repo in CE when backporting changes from EE

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/46239