Implement Proxy object [[Get]] internal method (#3604)

The algorithm is based on ECMA-262 v6, 9.5.8

JerryScript-DCO-1.0-Signed-off-by: Adam Szilagyi aszilagy@inf.u-szeged.hu

tests/jerry/es2015/proxy_create.js

@@ -29,13 +29,6 @@ try {
   assert(e instanceof TypeError);
 }
 
-try {
-  proxy.a;
-  assert(false);
-} catch (e) {
-  assert(e instanceof TypeError);
-}
-
 try {
   new Proxy(undefined, undefined);
   assert(false);