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.

This reverts commit 6c632dc6

Instead of querying the slots in the shadow DOM after the first render
just add all available slots to the light DOM.

This fixes missing filled slots when slots where added after the first render,
for example if they were used only after some property is set.