Integration with DOM Parts API

[lukewarlow]

This issue tracks the integration with the DOM Parts API while replaceChildren is not currently in the spec proposal Chromium has the following IDL change:

partial interface ChildNodePart : Part {
  [RaisesException] void replaceChildren((Node or DOMString or TrustedScript)... nodes);
};

If the DOM Parts API ever gets specced we should ensure this spec covers it, or that we update that spec accordingly depending on how trusted types is integrated with other specs in future.