Rename DOMUtils.isTplElementNode --> isTplOrExtToplevelNode

* The name was confusing me in a new patch I was working on.
  I wanted to process template or extension content and thought
  I had to add additional code, but realized that the poorly
  named helper was handling both template and extension content.

  In the past, we've had some discussions about this common pattern
  in Parsoid code where "template content" is used as a proxy for
  generated content -- either by templates or extensions but due to
  lack of a good name, we left the proxy name as is in most places in
  the codebase. As expected, after coming back to this code after
  a long time, I did get confused by the name.

Change-Id: Idb24adae3d3f6039856851cfc17e89b393cf6458