Add support for lookbehind in region regexes

Use the 'regexp2' library for lookahead and lookbehind in region
start and end regular expressions to support things like closing quotes
that aren't preceded by backslashes.

runtime/syntax/coffeescript.yaml

@@ -16,7 +16,7 @@ rules:
 
     - constant.string:
         start: "\""
-        end: "\""
+        end: "(?<!\\\\)\""
         rules:
             - constant.specialChar: "\\\\."
 
@@ -25,3 +25,4 @@ rules:
         end: "$"
         rules:
             - todo: "(TODO|XXX|FIXME):?"
+