feat: check duplicate comma-separated @keyframe selectors

[Tanujkanti4441]

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• I have read the contributing guidelines.

AI acknowledgment

• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

What is the purpose of this pull request?

To also check the duplicate @Keyframe selectors that are comma-separated.

What changes did you make? (Give an overview)

Added a loop to go through every selector in a rule instead of just the first one to enable checking comma-separated selectors as duplicates, and added related test cases also. Now rule no-duplicate-keyframe-selectors will also report cases such as:

@keyframes a {
	0%, 50% { top: 0; }
	50% { top: 1px; }
}

@keyframes a {
	from, 0% { top: 0; }
	50% { top: 1px; }
}

@keyframes a {
	entry 0%, { top: 0; }
	entry 0%, 50% { top: 1px; }
}

Related Issues

Fixes #459

Is there anything you'd like reviewers to focus on?

[DMartens]

Changes LGTM, thanks. As this is a feature leaving open for a second review.