Complete revamp of markdown-gfm

[fredck]

Suggested merge commit message (convention)

Feature (markdown-gfm): The markdown data processor has been totally revamped and updated to the most recent conversion libraries. Closes #5988.

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Additional information

This PR is used in production within GitHub Writer.

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The main updates related to this PR are the use of the most up-to-date libraries for markdown conversion:

• The marked library was 5 years old. Meanwhile it became a solid, well maintained and active project.
• The to-markdown library dies and reborn, now named Turndown. It is a very active and beautiful project, maintained by many and reaching a great level of quality.

I've also removed the libraries from inside our code, making them normal dependencies.

One important impact of the updated on the libraries is that many tests started to fail. That's because the libraries are much more standard oriented now and less interested on the markdown-madness of accepting whatever is given.

All in all, the quality of the implementation is definitely higher.

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Original PR: ckeditor/ckeditor5-markdown-gfm#29

[jodator]

Hi @fredck, thanks for contributing to the project!

Sorry, I couldn't resist 😉.

I had to do some code alignment to the what the team is used to so most of my changes are mostly cosmetics:

1. We don't use foo && foo.bar && doSomethingWithFooBar( foo ) due to readability issues. Unfortunately our ESLint allows that: Re-enable no-unused-expressions ESLint rule #7843.
2. I've moved RegExp and escape definitions out of TurndownService.prototype.escape definition - we can define them once, not per every TurndownService.prototype.escape call - improved memory management.
3. I've refactored URL matching to use String.matchAll() - IMO it is a nicer API over the RegExp.exec + do...while. So just a preference.
4. To ensure I don't broke anything I've added a test for escaping string between two URLs.

The PR is not ready to be merged yet since I need to fix API docs (I'll explain changes later) - but in short - you can't provide empty @module  jsdoc annotation - it must be a valid "path"-like entry, something like:

/** @module markdown-gfm/html2markdown/html2markdown */

[jodator]

ps.: The merge will happen soon, together with the rest of markdown-related PRs so it should be available in the upcoming release :todo:.

[jodator]

ps.: The merge will happen soon, together with the rest of markdown-related PRs so it should be available in the upcoming release 🎉.

[jodator]

The errors are on master as well :/ Merging as plugin has no own errors.