(Bug 64300) Deal with <nowiki/> escaping around url-links

* Stripped out existing helpers from computeDSR code and moved
  to DOMUtils for use in serializer.

* Renamed "stx": "protocol" data-parsoid flag to "stx": "magiclink"
  since that is a better name.

* Added several new parser tests for nowiki escaping for magic-links
  and url-links.

* One of them (Links 7b.) fails wt2html and html2html -- this
  requires a tokenizer fix (unrelated to this patch).

* Detecting the link-syntax for the serialized-form without the
  presence of syntax markers and rel attributes requires us to
  do the full set of tests that are currently done in the link handler
  to serialize links. Doing this well and without duplicate work
  requires extracting that code out of the link-handler and using
  it everywhere. Something for a future patch perhaps.

Change-Id: I60b26dc1dad42b834f2e2e746bd5946b4d7f8b8d
Co-authored-by: Subramanya Sastry
Co-authored-by: C.Scott Ananian
Merge "(Bug 64300) Deal with <nowiki/> escaping around url-links"