This PR changes the typedef for CustomElement to be CustomElementClass which changes from:

interface new CustomElement {
  new(): HTMLElement
}

to

interface new CustomElementClass {
  new(): CustomElement
//...
}

The CustomElement type then extends HTMLElement but adds all the optional callbacks that make up a Custom Element, e.g. ConnectedCallback.

This isn't necessary for the existing code, but it will enable us to have more safe guards in future as we work with these methods, because TS will now check that we're interacting with them correctly.