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/parser/js/js-parser-expr.c

@@ -1883,6 +1883,12 @@ parser_parse_unary_expression (parser_context_t *context_p, /**< context */
             {
               is_negative_number = !is_negative_number;
             }
+#if ENABLED (JERRY_BUILTIN_BIGINT)
+            else if (JERRY_LIKELY (context_p->token.extra_value == LEXER_NUMBER_BIGINT))
+            {
+              break;
+            }
+#endif /* ENABLED (JERRY_BUILTIN_BIGINT) */
             parser_stack_pop_uint8 (context_p);
           }
           while (context_p->stack_top_uint8 == LEXER_PLUS