Add TypeScript type-checking

[aduth]

🛠 Summary of changes

Adds TypeScript type-checking for existing JavaScript files.

As evidenced by the included changes, this helped highlight a couple legitimate issues:

• The test for ensuring all components are exported was not working as intended, and therefore there were several components not being exported
• The HTML_CodeSniffer test incorrectly assumed there would be an error argument passed in the callback, which doesn't exist

📜 Testing Plan

npm run typecheck