Implement special node types for language variant markup, so that they
display appropriately based on the currently-selected variant.
(Parsoid uses empty elements to represent this markup, so without this
patch anything in -{ ... }- is alienated and disappears.)
A follow-up patch will implement context items and inspectors to
allow editing these nodes. This patch is basic "read-only" support.
Depends on I4fcdebc2290ec35ba188f4c2e69d578791fbcd67 in Parsoid to
generate the appropriate markup, but this patch is safe to merge
independently.
Bug: T49411
Change-Id: Ie11e9301d2513bfe4a36036481cee9a047f46d37
