Compatibility issue with ext/json

removed JSON_parser.h header
removed utf8_decode.h header
error codes constants moved to php_json.h
- enum error_codes renamed to php_json_error_codes (typedef)
- add JSON_ERROR_UTF16 for invalid \uXXXX code (ill-formatted sorrogate pairs)
- JSON_ERROR_STATE_MISMATCH is not used
- macro JSON_PARSER_DEFAULT_DEPTH renamed to PHP_JSON_PARSER_DEFAULT_DEPTH
ext global error_code type changed from int to php_json_error_codes
fix: consider minus character for overflow check

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