Improve BigInt support (#4118)

This patch adds several small features:
- Support right shift with negative numbers
- Support exponentation operator
- BigInts can be enabled in ES5.1 mode
- Remove dead code from ecma_deref_bigint
- Support longer BigInt literals in the parser
- Fix various BigInt comparison issues
- Do not discard unary plus for BigInt constants

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

jerry-core/vm/opcodes-ecma-arithmetics.c

@@ -134,11 +134,13 @@ do_number_arithmetic (number_arithmetic_op op, /**< number arithmetic operation
         ret_value = ecma_bigint_div_mod (left_value, right_value, true);
         break;
       }
-      default:
+#if ENABLED (JERRY_ESNEXT)
+      case NUMBER_ARITHMETIC_EXPONENTIATION:
       {
-        ret_value = ecma_raise_common_error (ECMA_ERR_MSG ("Not supported BigInt operation"));
+        ret_value = ecma_bigint_pow (left_value, right_value);
         break;
       }
+#endif /* ENABLED (JERRY_ESNEXT) */
     }
 
     ecma_free_value (left_value);