Add export map and UMD build

[nickbabcock]

The UMD build should make it easier to host on a CDN for users to test out (and they aren't familiar or can't use a package manager).

The export map follows the guidelines here: https://nodejs.org/api/packages.html#package-entry-points

We now score 100% on skypack quality: https://docs.skypack.dev/package-authors/package-checks

[github-actions[bot]]

Size Change: 0 B

Total Size: 95 kB

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `dist/es/index.js` | 95 kB |

_{compressed-size-action}