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/module-import-01.js

@@ -31,6 +31,3 @@ assert (mod.f("str") === "str")
 
 var dog = new mod.Dog("Oddie")
 assert (dog.speak() === "Oddie barks.")
-
-// Pre-scanner regression test
-for (var tmp in {}) ;