feat: add int64 support

[PeterAlfredLee]

I have noticed there are several PRs about int64 support. However, they are modifying the existing code and change cjson from c89 to c99, which I think is not a good idea. In this PR, I have added a new option ENABLE_INT64. By default, it's turned off, and cjson is still working in c89 without any differences. Only with ENABLE_INT64 turned on, the int64 support will be enabled and c99 will be used.

Here are some details:

This commit adds int64 support for cjson. To enable int64, you need to define the macro ENABLE_INT64. To make it more clear, you need to build cjson like this:

mkdir build
cd build
cmake -DENABLE_INT64=ON ..
make

This implementation changed the type of valueint in struct cJSON: from int to long long(int64), and added a new flag cJSON_IsInt64. For a int64 cJSON item, the value of item->type would be cJSON_Number | cJSON_IsInt64.

The existing functions parse_number and print_number can handle int64 now.

Considering I have added a new type cJSON_IsInt64, some new functions have been added:

CJSON_PUBLIC(long long *) cJSON_GetInt64NumberValue(cJSON * const item)
CJSON_PUBLIC(cJSON_bool) cJSON_IsInt64Number(const cJSON * const item)
CJSON_PUBLIC(cJSON *) cJSON_CreateInt64Number(long long integer)
CJSON_PUBLIC(cJSON*) cJSON_AddInt64NumberToObject(cJSON * const object, const char * const name, const long long integer)
CJSON_PUBLIC(long long) cJSON_SetInt64NumberValue(cJSON * const object, const long long integer)

And some existing functions are also adjusted for int64, including parse_number, cJSON_SetNumberHelper, print_number, cJSON_CreateNumber, cJSON_Compare and compare_json(in cJSON_Utils).

Besides the code changes, we have also added some testcases for int64. These tests will run if ENABLE_INT64 is turned on.