Added human lang missing audit rule

@alice - PTAL
This is fine in this case, but note that a more robust way to do this is to check the prototype chain. (For example, for headers we don't want to check against every single tag from h1 to h6 because all of them cause the same type of object to be created - HTMLHeaderElement.)

https://wildlyinaccurate.com/understanding-javascript-inheritance-and-the-prototype-chain is a pretty good explanation of how the prototype chain works, with some good examples of using devtools to explore objects.

The way we check whether a particular proto is in the prototype chain is using the instanceof operator: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof

We do this in a bunch of other places in the code, e.g. https://github.com/GoogleChrome/accessibility-developer-tools/blob/master/src/js/AccessibilityUtils.js#L307

Note that we always use element.ownerDocument.defaultView.<ElementType> - this annoying verbosity is because one instance of the <ElementType> prototype exists in each frame, and if this script is communicating across frames (as it does in the extension), the <ElementType> prototype in the frame that the script is executing might not be the same as the one in the frame that the element exists in. This is explained in slightly more detail in the MDN article on instanceof. ...Welcome to JavaScript :)