Clears visual token on second backspace

What does this MR do?

Creates a closure for the checkForBackspace event handler in the FilteredSearchManager and removes a visual token only if the user hits backspace twice. This way an ancestor token won’t get removed immediately when a user removes the last character of the current token as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/38596

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

Why was this MR needed?

It improves the usability of clearing tokens in the issue search component as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/38596

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #38596 (closed)