While this will never work with old Edge it makes things easier during testing
where we can't set an id on the script tag. And we still don't know if supporting
old Edge is even needed.

This is a common source for errors and we currently fall back to a relative path
which happens to work locally but not on the server.

This splits things into two phases, initAssetBaseURL() will try to set
a module wide base url for all assets.

The first code to use it successfully will define the asset base url for
all future getAssetURL() calls.

So that rollup can do tree shaking and report import errors.

So rollup doesn't have to convert them (and potentially fail)

In case the browser doesn't support custom elements and shadow dom we display
an error instead of the element.

This assumes the JS code executed thus far didn't lead to errors, so this only works
for semi-modern browers.