Method is_internal

include/stringzilla/stringzilla.hpp:3249–3249

    bool is_upper() const noexcept { return !empty() && contains_only(ascii_uppercase_set()); }
    bool is_printable() const noexcept { return empty() || contains_only(ascii_printables_set()); }
    bool is_internal() const noexcept { return sz_string_is_on_stack(&string_); }

#pragma region Character Set Arguments