Add ElementInternals polyfill

[alexgagnon]

Feature

What

I'd like to request the ElementInternals polyfill (https://www.npmjs.com/package/element-internals-polyfill) be added to polyfill.io

Details

ElementInternals allows for attaching inputs in the shadow DOM to forms. Currently all browsers except for Safari support it: https://caniuse.com/?search=elementinternals