Implement var statement pre-scanning. (#3103)

The patch also checks whether pre-scanning is successful when scanning is,
so no need for explicit pre-scanner checks anymore.

JerryScript-DCO-1.0-Signed-off-by: Zoltan Herczeg zherczeg.u-szeged@partner.samsung.com

tests/jerry/es2015/class.js

@@ -236,6 +236,3 @@ assert (G.get() == 11);
 assert (G.set() == 12);
 G.constructor = 30;
 assert (G.constructor === 30);
-
-// Pre-scanner regression test
-for (var tmp in {}) ;