Conversion to TypeScript

[bmish]

Follow-up to:

• Add TypeScript type-checking to all rules #241

Converting to actual TypeScript isn't really necessary since we already have the type-checking with jsdocs and published types now, but this is more of a nice-to-have.

I started this in the following branch:

• https://github.com/qunitjs/eslint-plugin-qunit/compare/main...bmish:ts?expand=1

Related example:

• feat: migrate package to TypeScript and publish types eslint-community/eslint-plugin-eslint-plugin#534