Enable MergeableSelector in scss-lint

What does this MR do?

• Enables MergeableSelector in scss-lint.
• Fixes all MergeableSelector violations.

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

Yes, all changes should be checked by multiple reviewers.

Why was this MR needed?

To improve SCSS maintainability and consistency.

This rule is relatively likely to tricky as nests tend to be deeper but still effective for various authors.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Part of #23485 (moved)