WIP: ESM + CJS dual support

[julrich]

Some initial work to enable ESM usage of the package. Also keeps CJS for backwards compatibility.
The changes are in part based on this article by Anthony Fu: https://antfu.me/posts/publish-esm-and-cjs

Additionally, tsup is used to dual-bundle: https://github.com/egoist/tsup

Test suite still runs, and is all green.

Still though, more testing is needed for:

• CJS usage... ESM works, which is what we're using, but CJS is kind of untested (though I think the test suite should access the CJS version, and all the tests still pass)
• Browser usage. I think this would be best tested through https://github.com/bcherny/json-schema-to-typescript-browser? Haven't dove in there yet, though

[bcherny]

Thanks for the contribution, but I'm a little confused what's going on in this PR. Why are we relying on your @kickstartds/json-schema-to-typescript fork?

Let's continue the discussion in #551.