[alignment with standards] refactor AOM properties to be surfaced via ElementInternals

[caridy]

Description

Today, we are exposing AOM properties via this.template, this is changing in the spec, and it now will be exposed via createdCallback, who receives an argument internals (instance of ElementInternals). Changes are fairly simple in LWC, but there are some users using AOM properties from template that might get affected by this. Maybe we can keep both references for now, until this settles after 218.

Additionally, the introduction of the new hook is also important, which means that they now have the ability to do things right after the constructor finishes. I'm not entirely sure about the semantics of this hook.

[caridy]

Backpointer: https://github.com/WICG/aom/pull/129/files

[caridy]

The initial implementation was first attempted in #754, but eventually closed in favor of going slowly considering that there are still things TBD about that proposal. For now, we have removed all the shadowRoot patching for AOM in #794

[caridy]

There is a new PR for the Internals specification:

whatwg/html#4658

Once that is merged, we can start implementing that on our end.

[nolanlawson]

@caridy What is left to do here? Is this just about implementing attachInternals() and the various FACE callbacks now?

Related: #3000

[nolanlawson]

Related: #1023