Expose maximum size for error buffer in header

I felt uncomfortable just having a magic number in my code, having to remember to update it whenever the value changes in `json.c`, so I added this definition in `json.h`. Let me know if it needs to be in all caps (json_relaxed_commas was in all lowercase) or if it needs to be moved to a different line in the header file (I just placed it above `json_parse_ex` since that's the function that takes the error buffer parameter).