This generalizes the features of the events module and should replace it in the long run.

It allows simple pub/sub and retained events without a centralized instance.

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 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.