Add and enable the validate-changelog-entry rule for validating Markdown-based changelog entries.

[filipsobol]

🚀 Summary

• Add and enable the validate-changelog-entry rule for validating Markdown-based changelog entries.
• Migrate CI from Machine executors to Docker executors.

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📌 Related issues

• See Create a rule for checking changelog entry files ckeditor5#18777

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💡 Additional information

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Error highlighting

I implemented this rule so that it tracks the positions of invalid entries rather than highlighting the entire frontmatter block as invalid. This makes it much easier to locate the issue.

output.mp4

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How to test

1. Clone this repo into the external folder and check out the ck/18777-add-rule-to-validate-changelog-entry branch.
2. Check out the ck/18777-lint-changelog-entries branches in the core and commercial repositories (core PR, commercial PR).
3. Run yarn reinstall.
4. Run yarn nice in both the core and commercial repositories, and experiment with the generated changelog file. Add valid and invalid values, remove values, etc., to verify that all cases are covered by the rule and that it doesn't break under any circumstances.

[filipsobol]

Unlike other tests in this project, this one uses ESM. This is because otherwise ESLint fails to load the markdown/gfm language.

However, because all tests are loaded using require() I had to update our CI configuration to use Node 22.12, which added support for require(esm). This is the same change we recently implemented in other repositories.

[filipsobol]

Explained in https://github.com/ckeditor/ckeditor5-linters-config/pull/71/files#r2192714688

[przemyslaw-zan]

Perhaps we could follow DRY principle, and extract common values to a common key, and create a function that would return joined list? Then again, its just twice, so I'm, not really insisting here.

[filipsobol]

IMO that will overcomplicate a pretty simple bit of code. Pulling things out and adding a function would just add unnecessary abstraction without much real benefit here.