no-empty-url: check urls from definitions

[Xunnamius]

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)
• If applicable, I’ve added docs and tests

Description of changes

First, thank you for remark and the unified ecosystem!

This is a small fix to allow remark-lint-no-empty-url to also consider the URLs of Definition nodes when linting. When I first used this rule, I assumed that, because of the name, it would apply to all URLs. However, after using remark-reference-links, any mistakenly empty URLs normally caught by this rule were transformed into mistakenly empty definitions that were no longer caught by this rule, which came as a surprise to me.

Along with updated tests and documentation, I'm currently using my forked version across several projects if you want to test it out real quick:

npm install --save-dev https://xunn.at/remark-lint-no-empty-url
npx remark -o --no-config --use remark-lint-no-empty-url file-with-empty-definition.md

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (8f222f3) compared to base (6ca7eef).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #299   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           73        73           
  Lines         8475      8486   +11     
=========================================
+ Hits          8475      8486   +11     

Impacted Files	Coverage Δ
packages/remark-lint-no-empty-url/index.js	100.00% <100.00%> (ø)

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[wooorm]

Thank you, good idea!!