[scoped-custom-element-registry] Support deferred scoped registry initialization

[tomherni]

Description

Add support for deferred scoped registry initialization. This means to support initialize() on the CustomElementRegistry interface that allows for associating a scoped registry with an already attached shadow root.

Motivation

Associating scoped registries after shadow roots were created can be useful in general. But it is needed to support scoped elements rendered on the server with Declarative Shadow DOM (that uses shadowrootcustomelementregistry).