Fix unit-no-unknown false positives for the second and subsequent image-set() with x descriptor

[chalkygames123]

Which issue, if any, is this issue related to?

Fixes #6721

Is there anything in the PR that needs further explanation?

I'll do some refactoring later on, as @ybiquitous suggested (it may require a few weeks).

[changeset-bot]

⚠️ No Changeset found

Latest commit: 451b127

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[romainmenke]

@chalkygames123 Thank you so much for this change 🙇

I was working on the migration to a new parser for media queries and in doing so I updated this same rule. With that change the issue you are solving here no longer exists.

I added your tests from your pull requests here : #6879

[chalkygames123]

@romainmenke Cool, that's much more elegant. Thank you for your effort.