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-lexer.c

@@ -1474,7 +1474,7 @@ lexer_parse_number (parser_context_t *context_p) /**< context */
 #endif /* ENABLED (JERRY_BUILTIN_BIGINT) */
 
   length = (size_t) (source_p - context_p->source_p);
-  if (length > PARSER_MAXIMUM_IDENT_LENGTH)
+  if (length > PARSER_MAXIMUM_STRING_LENGTH)
   {
     parser_raise_error (context_p, PARSER_ERR_NUMBER_TOO_LONG);
   }