Implement the `HTMLOrSVGElement` WebIDL

[nolanlawson]

The HTMLOrSVGElement mixin defines a few properties and methods:

• dataset
• nonce
• autofocus (#32170)
• tabIndex
• focus()
• blur()

These are partially implemented in Servo – dataset, focus(), and blur() as of this writing. But to match the spec, they should be part of this mixin, so that they can apply to both HTMLElements and SVGElements, not just HTMLElements.

Here is the full WebIDL for completeness:

interface mixin HTMLOrSVGElement {
  [SameObject] readonly attribute DOMStringMap dataset;
  attribute DOMString nonce; // intentionally no [CEReactions]

  [CEReactions] attribute boolean autofocus;
  [CEReactions] attribute long tabIndex;
  undefined focus(optional FocusOptions options = {});
  undefined blur();
};