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-03.js

@@ -22,6 +22,3 @@ assert(i.incr() === 6);
 assert (aa === "a");
 assert (x === 42);
 assert (c_(x) == 84);
-
-// Pre-scanner regression test
-for (var tmp in {}) ;