Optimize jsmntok_t memory footprint via configurable field types

[metalalive]

Description

This patch introduces build-time configuration macros for jsmntok_t field types to reduce memory usage on
resource-constrained targets as described in the issue 239.

Changes

• added macros for token fields (e.g. JSMN_TOKEN_START_TYPE) with default fallback to int.
• refactored jsmntok_t struct to utilize these macros, allowing users to override types (e.g., short, int8_t) before
  including jsmn.h.
• moved jsmntype_t type to the end of the struct and applied bit-field sizing (: 8) to improve struct packing and alignment.
• updated README.md with usage examples for memory-constrained environments.

Impact

Size of a jsmntok_t instance can be reduced , enabling more token array items in limited RAM environments without changing the API.

Test Result

I don't find any CI flow (GitHub Action ?) in this repo , allow me to post my test result at here .

make test with custom config.mk

cc -g -DJSMN_TOKEN_START_TYPE=char -DJSMN_TOKEN_END_TYPE=char -DJSMN_TOKEN_SIZE_TYPE=char  test/tests.c -o test/test_default
./test/test_default

PASSED: 16
FAILED: 0
cc -DJSMN_STRICT=1 -g -DJSMN_TOKEN_START_TYPE=char -DJSMN_TOKEN_END_TYPE=char -DJSMN_TOKEN_SIZE_TYPE=char  test/tests.c -o test/test_strict
./test/test_strict

PASSED: 16
FAILED: 0
cc -DJSMN_PARENT_LINKS=1 -g -DJSMN_TOKEN_START_TYPE=char -DJSMN_TOKEN_END_TYPE=char -DJSMN_TOKEN_SIZE_TYPE=char  test/tests.c -o test/test_links
./test/test_links

PASSED: 16
FAILED: 0
cc -DJSMN_STRICT=1 -DJSMN_PARENT_LINKS=1 -g -DJSMN_TOKEN_START_TYPE=char -DJSMN_TOKEN_END_TYPE=char -DJSMN_TOKEN_SIZE_TYPE=char  test/tests.c -o test/test_strict_links
./test/test_strict_links

PASSED: 16
FAILED: 0

memory check with the test executable

valgrind --leak-check=full ./test/test_default 
==39382== Memcheck, a memory error detector
==39382== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==39382== Using Valgrind-3.22.0 and LibVEX; rerun with -h for copyright info
==39382== Command: ./test/test_default
==39382== 

PASSED: 16
FAILED: 0
==39382== 
==39382== HEAP SUMMARY:
==39382==     in use at exit: 0 bytes in 0 blocks
==39382==   total heap usage: 37 allocs, 37 frees, 1,504 bytes allocated
==39382== 
==39382== All heap blocks were freed -- no leaks are possible
==39382== 
==39382== For lists of detected and suppressed errors, rerun with: -s
==39382== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)