Parse BlockString (#194)

Adds the BlockString rule to the String regex parser.

I noticed there are no unit tests for the parser in this repo, so tested with https://regexr.com/3iclo

Author: leebyron

packages/parser/src/Rules.js

@@ -42,7 +42,7 @@ export const LexRules = {
   Number: /^-?(?:0|(?:[1-9][0-9]*))(?:\.[0-9]*)?(?:[eE][+-]?[0-9]+)?/,
 
   // Note the closing quote is made optional as an IDE experience improvment.
-  String: /^"(?:[^"\\]|\\(?:"|\/|\\|b|f|n|r|t|u[0-9a-fA-F]{4}))*"?/,
+  String: /^(?:"""(?:\\"""|[^"]|"[^"]|""[^"])*(?:""")?|"(?:[^"\\]|\\(?:"|\/|\\|b|f|n|r|t|u[0-9a-fA-F]{4}))*"?)/,
 
   // Comments consume entire lines.
   Comment: /^#.*/,