Роман Донченко authored Apr 30, 2019 and Christian Hergert committed Apr 29, 2019

* Allow them to be multiline.

* Add highlighting for escape sequences. This necessitates splitting
  the "string" context into "single-quoted-string" and
  "double-quoted-string", as the escaping rules are different.

* Allow empty quoted strings ("") to be highlighted correctly. The previous
  end regex prevented the closing quote in this case to be recognized
  as such, as it required it to be preceded by at least one character.