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.

[lumirlumir]

Looks so good to me. Thanks 👍

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

[nzakas]

@jaymarvelz please check the merge conflict.

[mdjermanovic]

LGTM, thanks! Leaving open for @nzakas to verify, as requested.

[nzakas]

LGTM. Thanks!