switch to rollup-plugin-terser

[gavinr]

Switching from rollup-plugin-uglify to rollup-plugin-terser.

This PR replaces #1235, but @jwasilgeo's comment still applies:

If this change goes through, then I suggest we also think about updating these plugins' npm deps that rely on esri-leaflet's own Rollup config:

• https://github.com/Esri/esri-leaflet-vector
• https://github.com/Esri/esri-leaflet-geocoder
• https://github.com/Esri/esri-leaflet-cluster
• https://github.com/Esri/esri-leaflet-heatmap