chore: re-write some files to typescript

[shadowusr]

What's done?

• re-written a few files to typescript as a preparation step for playwright support
• fixed mocha tests running: tests in jsx files were not being launched before

Why?

• to introduce strong types and be able to write new code in TS

⚠️ Notes

• package-lock.json was reverted in the last commit to not clutter PR diff and that's why checks are failing
• every commit is atomic, meaning all checks pass on every commit but the last one
• might be a good idea to review each commit separately: in the final diff some moved files appear as completely removed/added, but when viewing commits separately you can see what's been truly changed