[labs/scoped-registry-mixin] Provide a method that allows lazy scoping of elements

Nobody has claimed this yet.

### Should this be an RFC?

- [X] This is not a substantial change

### Which package is this a feature request for?

@lit-labs/scoped-registry-mixin

### Description

In some situations you may want to use a component in your templates that is not already loaded at the moment of defining the scoped elements map. It would be nice to provide a defineScopedElement method to define scoped elements at any time.

This is how the @open-wc library implemented a way to lazily created scoped elements:

https://open-wc.org/docs/development/scoped-elements/#lazy-scoped-components

```js
this.defineScopedElement('my-button', MyButton)
```

### Alternatives and Workarounds

There doesn't appear to be a workaround in this library to achieve this.

Start in the @lit-labs/scoped-registry-mixin package by reading how the scoped elements map is defined and how elements are currently registered. Compare the requested behavior with the linked open-wc lazy-scoped-components approach. Done means a defineScopedElement method can register an element after the scoped map is defined, with tests covering that use case.

Written by the indexing model from the issue text.