remark-lint: `no-undefined-references`: support regex in `allow` option

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)

Problem

The Markdown I’m linting contains undefined references because they are implicitly resolved by the documentation library I’m using (in my case the MkDocs extension mkdocstrings via its cross-references feature). I would like to support allowing undefined references according to a regex pattern, typically something like ^mylib\. (which is a Python root package prefix in my case).

Solution

For this to work, the allow option would need to be extended to support regex patterns.

I imagine the options object could be extended to this type (TS syntax):

type Options = {
  allow: Array<string | { pattern: string }>
}

If this proposal gets accepted, I’d be happy to submit a PR.

Alternatives

Without supporting a regex pattern, I can only disable the linter:

<!-- lint disable no-undefined-references -->
[`Object 1`][full.path.object1]
<!-- lint enable no-undefined-references -->

But that’s a bit tedious to do everywhere in the docs.