feat: add allowLabels option to no-missing-label-refs

[jaymarvelz]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

This PR adds a new allowLabels option to no-missing-label-refs, allowing users to intentionally permit specific labels without definitions.

What changes did you make? (Give an overview)

• Updated the rule logic to skip reporting missing references whose labels are listed in allowLabels.
• Documented the option with example configuration and usage.
• Added tests to cover valid ignores and non-ignored labels still reporting.

Related Issues

Fixes #449

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

[lumirlumir]

Thanks for working on it.

Just one thought: Would anyone be open to renaming the option to allowLabels here?

Since most rules in markdown use allow for array option names, I think this change would help make the option names more consistent.

markdown/src/rules/no-duplicate-definitions.js

Lines 70 to 71 in c790317

	allowDefinitions: ["//"],
	allowFootnoteDefinitions: [],

markdown/src/rules/no-empty-definitions.js

Lines 89 to 90 in c790317

	allowDefinitions: ["//"],
	allowFootnoteDefinitions: [],

markdown/src/rules/no-html.js

Line 70 in c790317

allowed: [],

markdown/src/rules/no-unused-definitions.js

Lines 71 to 72 in c790317

	allowDefinitions: ["//"],
	allowFootnoteDefinitions: [],

[jaymarvelz]

Yeah, that makes sense 👍. I went ahead and updated the option name.

[nzakas]

In your original issue you mentioned wanting to use ignoreLabels. Why the change?

[jaymarvelz]

I initially called it ignoreLabels, but @lumirlumir suggested being consistent with other rules, so I renamed it. Would you like me to revert the change?

[lumirlumir]

I've asked @jaymarvelz (and other team members) for their opinion on #513 (comment), how about renaming it to allow for consistency across the Markdown rules.

If you feel ignore is more appropriate here, please disregard my suggestion and feel free to keep ignore.

[snitin315]

allowLabels amkes sense to me 👍🏻

[lumirlumir]

Looks so good to me. Thanks 👍

Would like @nzakas to verify (including the option name) before merging.