fix: detect prefixed @keyframes in no-duplicate-keyframe-selectors

[thecalamiity]

Prerequisites checklist

• I have read the contributing guidelines.

What rule do you want to change?

no-duplicate-keyframe-selectors

What change do you want to make?

Generate more warnings

How do you think the change should be implemented?

A new default behavior

Example code

@keyframes test {
      0% { opacity: 0; }
      0% { opacity: 1; }
 }
 
@-webkit-keyframes test {
      0% { opacity: 0; }
      0% { opacity: 1; }
 }

What does the rule currently do for this code?

It only detects duplicate keyframe selectors in unprefixed @keyframes blocks. Prefixed blocks like @-webkit-keyframes, @-moz-keyframes, or @-o-keyframes are ignored, so duplicates there are not reported.

What will the rule do after it's changed?

It will also detect duplicate keyframe selectors in vendor-prefixed @keyframes blocks, ensuring duplicates are reported consistently across prefixed and unprefixed animations.

What changes did you make? (Give an overview)

• Broadened Atrule matchers (enter/exit) to use: /^(-(o|moz|webkit)-)?keyframes$/i
• Add tests to ensure detection works for prefixed forms.

Related Issues

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

[nzakas]

Thanks for the PR. Because this is also a bug report, can you please update the description with the same fields as the bug report issue template?

[thecalamiity]

Sorry about that. Updated now.

[nzakas]

LGTM. Thanks!