Feat/support dual esm cjs

[LiamMartens]

What is the purpose of this pull request? (put an "X" next to an item)

• [ ] Documentation update
• [ ] Bug fix
• [ ] Feature
• [ ] Code style update (formatting)
• [ ] Refactoring (no functional changes)
• [ ] CI related changes
• [x] Other, please explain: Fixes problem with dual publishing of library and NodeNext moduleResolution (#191)

What changes did you make? (Give an overview)

Updated the build

Is there anything you'd like reviewers to focus on?

As far as I can tell, the new artifacts are functionally identical. Both the ESM and CJS outputs support default importing/requiring.

import mitt from 'mitt' // works
const mitt = require('mitt') // works