added unsigned char casting for errors in esp-idf building system

[FBEZ]

While porting tinyexpr to [Espressif's ESP-IDF](https://github.com/espressif/esp-idf), I encountered a compiler warning related to the use of signed char types in the isalpha and isdigit macros:

array subscript has type 'char' [-Werror=char-subscripts]

This appears to be a platform-specific issue. I wasn’t able to reproduce the same warning on macOS, even when explicitly setting the compiler flags:

-fsigned-char -Wchar-subscripts -Werror=char-subscripts

Since I’m considering using your nice project as an example of how to port a C library as an ESP-IDF component, I wanted to contribute this small fix and share it back with you—this way, I can reference your repo directly in my write-up.

Thanks for maintaining such a clean and minimal implementation! :)