Fix backslashes in backquotes

This fixes highlighting of raw string literals such as *a single backslash surrounded by backticks*. I'm unsure about the exact  Emacs version where this was made possible.

See also https://stackoverflow.com/questions/25089090/emacs-d-mode-cannot-handle-backquoted-backslashes/25150600?noredirect=1#comment39366860_25150600