Build: use JS for eslint config

[timmywil]

• this makes it easier to use this config in esm eslint.config.js

To lint the config itself without changing the config, I've moved it to index.js and added a .eslintrc.js for this package that extends index.js and sets node: true.

Helps with jquery/jquery#5297

[mgol]

Does it change anything beyond the fact we can lint the file within this repo?

We're still not doing that, BTW. Please expand npm test to lint the whole project.

[timmywil]

Yes, you cannot import JSON in eslint.config.js without a warning. Switching to JS simplifies things.