Triple strings are now distinct from docstrings, triple strings
only preceeded by blanks on the line are considered docstrings.
Avoid highlighting of the closing marker using a lookahead, this
is not fully correct as it will break on a double quote triple
docstring containing a single quote triple string but that seems
improbable enough; if we encounter this in the wild we can split
the two docstring formats into separate regions.
Reference highlighters allow for potential mutual recursion between
highlighters. This is usually fine, but if the recursion happens on
the same buffer range, it means we will recurse infinitely.
Fixes#1920
