What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• Others, please explain:

Overview of changes:

• tighten eslint ignore rules for the lib/markdown-it directory to only ignore adapted source files largely unchanged from the original source material
• make the necessary changes to comply with the rules

Anything you'd like to highlight / discuss:
na

Testing instructions:
na

Proposed commit message: (wrap lines at 72 characters)
Eslint appropriate lib/markdown-it files

The lib/markdown-it directory in the core package is entirely ignored
by our eslint rules.

Let's tighten the eslint ignore rules to only ignore adapted source
files that are largely unchanged from the original source.

This allows easily referencing the original source material if needed.

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No blatantly unrelated changes
• Pinged someone for a review!