Since it's not clear if IRIs should be unique in the API (also with subclasses etc)
just stop usingn them for now, to avoid name ID clashes.

In case the scoped registry polyfill is loaded we need to create
elements inside of the shadowroot.

getShadowRootDocument() returns the polyfilled shadowroot, or in case
the polyfill is not loaded just return the global document, which works
in case there are no scoped registries.

In case we get something like https://github.com/open-wc/open-wc/pull/2389
this will be usefull. Also it makes IDE completion work + eslint.