src/parser.pegjs: Replace "\0" with "\x00"

Fixes the following JSHint error:

  ./src/parser.js: line 2820, col 44, Bad escapement.

src/parser.js

@@ -2817,7 +2817,7 @@ PEG.parser = (function(){
           pos = pos1;
         }
         if (result0 !== null) {
-          result0 = (function() { return "\0"; })();
+          result0 = (function() { return "\x00"; })();
         }
         if (result0 === null) {
           pos = pos0;

src/parser.pegjs

@@ -294,7 +294,7 @@ simpleEscapeSequence
     }
 
 zeroEscapeSequence
-  = "\\0" !digit { return "\0"; }
+  = "\\0" !digit { return "\x00"; }
 
 hexEscapeSequence
   = "\\x" h1:hexDigit h2:hexDigit {