Add `class` as an alias for `className`

[Psychpsyo]

What is the issue with the DOM Standard?

Currently, to access the class attribute of an element in Javascript, you need to use element.className instead of just element.class. This is unintuitive and inconsistent with other attributes, and there is little reason for it because class is no longer a reserved word.

Note: This is just to mirror issue #9379 in the HTML repo. See it for more details and discussion.