Add keyframe-block-no-duplicate-selectors

[mattxwang]

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

Closes #5991.

Is there anything in the PR that needs further explanation?

Still newish to the overall codebase! A couple of questions I had (and of course, general feedback is much appreciated):

1. Should I explicitly be handling case-insensitivity? Alternatively, should I document that the rule is case-sensitive (as of now)
2. How comprehensive are the test cases and documentation? I'm not sure if I'm missing any common cases where this would be a problem.
3. I've added vendored prefixes to the regex for finding keyframes as well as in the test cases - is this good practice?

[mattxwang]

For code coverage - is there a specific way I should be testing the rule with invalid options? That's the only uncovered line I have.

[ybiquitous]

@mattxwang Thanks for creating the pull request. 👍🏼

Additionally, it should be better to support end positions for warnings.
See also #5694 (comment)

[jeddy3]

@mattxwang Thank you! Looking good so far.

As @ybiquitous said, we'll want to add column and line numbers (including end ones) to the reject tests.

I've requested a few changes myself.

[jeddy3]

Should I explicitly be handling case-insensitivity?

Yes. Let's add @keyframes foo { from {} FROM {} } as a reject test.

How comprehensive are the test cases and documentation?

They're comprehensive enough, thank you.

I've added vendored prefixes to the regex for finding keyframes as well as in the test cases - is this good practice?

Yes, LGTM.

For code coverage - is there a specific way I should be testing the rule with invalid options?

You can ignore the coverage warning.

[mattxwang]

Thank you @jeddy3 and @ybiquitous for the in-depth feedback - end positions and non-standard syntax slipped my mind! I've pushed changes to address the feedback; let me know if you have any other thoughts.

[ybiquitous]

@mattxwang Nice! LGTM 👍🏼

[jeddy3]

@mattxwang Thanks for making the changes. LGTM.

[jeddy3]

• Added: keyframe-block-no-duplicate-selectors rule (#6024).