[13.x] Optimize Str::ucfirst and Str::lcfirst using native PHP 8.4 functions

[bunyaminbilenkaratas]

This PR optimizes the Str::ucfirst and Str::lcfirst methods by utilizing PHP 8.4's native mb_ucfirst and mb_lcfirst functions when they are available in the environment.

Why this matters:
Currently, these methods extract the first character using substr, mutate it (upper/lower), and concatenate it back with the rest of the string. Utilizing the native C-implemented multi-byte functions significantly reduces string segmenting overhead, avoids intermediate memory allocations, and provides a cleaner approach.

Backward Compatibility:
Backward compatibility is fully maintained for older PHP versions via a function_exists fallback.

All existing Str tests pass successfully.