This combines URLs similar to an URL join, but assumes the
base URL is the root path instead of the host.

This is useful for combining API paths with API server entry points.

This has proper support for rollup while providing the same API.

This is what open-wc recommends now and uses in their @open-wc/testing package

This way we can change the full path/namespacing at a later point.

We now expect the users to pass the right config to vpu-auth.
Only the API endpoint config remains which we haven't figured out
how to deal with in all cases.

We no longer need to support old edge, so we can just use
import.meta.url everywhere which doesn't any initialization.

In case we try to register the element multiple times under the same name
just do nothing. This allows us to not register things by default and
register under the same name in multiple places where needed without
coordination.

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.