Multiple IIIF image viewers on a single page broken in 4.0

[zerocrates]

See https://forum.omeka.org/t/seadragon-redeclaration-error/16594

The script for loading OpenSeadragon was changed to use const, but in the global scope, so loading that script more than once results in the const being declared again and reassigned, which is an error.

We could change the declarations here to var/let to just avoid the specific issue, introduce a surrounding scope so the consts don't conflict, or rewrite it so it's a function that's loaded as an external script that's either just called by each viewer, or is triggered on all viewers on load.