fix: replace deprecated String.prototype.substr() (#59)

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

lib/to-batch-syntax.js

@@ -44,6 +44,6 @@ function replaceDollarWithPercentPair (value) {
       startIndex = dollarExpressions.lastIndex
     }
   } while (dollarExpressions.lastIndex > 0)
-  result += value.substr(startIndex)
+  result += value.slice(startIndex)
   return result
 }