Release ESM build in dist

[gavinr]

Describe the problem

An ESM file is not included in the Esri Leaflet release:
https://unpkg.com/browse/esri-leaflet@3.0.8/dist/
... but would it be good if we did? I think this would allow users to import ESM directly from CDNs like Unpkg.

Describe the proposed solution

Leaflet releases an ESM build in their dist release. They started doing it at v1.3.2:

• https://unpkg.com/browse/leaflet@1.3.2/dist/ (compare to https://unpkg.com/browse/leaflet@1.3.1/dist/)
  

I have not looked into how they are generating it but I assume we could do a similar thing.

Alternatives considered

A workaround is to use the Skypack CDN like this: https://codepen.io/pen?template=wvoKwEv , but adding the ESM file ourselves would give added flexibility.

Additional Information

No response

[patrickarlt]

This sounds good to me.

[jgravois]

at this point i don't even remember how far i got with it, but you can find some prior art here 🎨

https://github.com/Esri/esri-leaflet/tree/es / 9d90ea5