[1.x] Switch to ESM

[danburzo]

Since it's a breaking change, this is planned for the 1.x release.

Switch to ES modules for all internal tooling, and rethink exports by:

• Switching to ESM by default;
• Providing legacy CJS / IIFE builds.

Further reading:

• Pure ESM package
• Get ready for ESM

[danburzo]

Eleventy currently doesn't like type: module in package.json, need to figure it out.

[danburzo]

Fixed on 1.x, the Eleventy thing was a non-issue (misread the docs).