Add customized built-ins via elementInternals

[anaskim]

Specification

WHATWG issue: whatwg/html#11061
OpenUI issue tracking initial discussions and WHATWG resolution to accept elementInternals.type: openui/open-ui#1088

Description

elementInternals.type supports the creation of custom elements that inherit the behaviors and properties of native HTML elements. When elementInternals.type is set to string values that represent native element types, custom elements can leverage the built-in functionality of standard elements while extending their capabilities to meet specific needs.

Documentation

Explainer: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/ElementInternalsType/explainer.md

Browser support

Starting to prototype in Chromium. Feature: https://chromestatus.com/feature/5111079237320704

[captainbrosset]

We'll have to think about how to handle this together with the existing customized-built-in-elements feature. This new proposal, based on elementInternals is a new approach to do customized built-in elements. The previous approach, which isn't in Safari, and will never be according to their position, is based on the is attribute.