feat: package exports update

[brandonjpierce]

This largely based on the following content & recommendations:

• https://github.com/frehner/modern-guide-to-packaging-js-library
• https://github.com/wooorm/package-exports
• https://github.com/arethetypeswrong/arethetypeswrong.github.io

Note that we are intentionally only targeting ESM and will not support CJS or UMD.