Add pkg.unpkg

[tomwayson]

Expected behavior

I expect to be able to load esri-loader from UNPKG like so:

<script src="https://unpkg.com/esri-loader"></script>

Actual behavior

I think this is related to https://github.com/Esri/arcgis-rest-js/issues/420

[tomwayson]

You can now load esri-loader using the above URL - the reason it wasn't working before was due to a bug in their CDN.

However, we should still do this.

[andygup]

Closing since this package is scheduled for deprecation at 4.29.