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

## Description

Add support for deferred scoped registry initialization. This means to support [`initialize()`](https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry/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`](https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/template#shadowrootcustomelementregistry)).