refactor(lib): fixing ts issues

[H4ad]

Changes

Fixes #11

Also, I added tsup to support dual package publish (commonjs & esm).

Checklist

• [x] Apply one of following labels; major, minor, patch or skip-release
• [x] I've updated the documentation, or no changes were necessary
• [x] I've updated the tests, or no changes were necessary

[kibertoad]

Merging, as it's better to postpone any runtime code changes until we have tests running, we can address class mutation later. I'll create a ticket.