Support for `resize` [perhaps also `capacity` and `reserve`] for cista::generic_string/basic_string

[ChemistAion]

There is a usage pattern for string like types when internal buffer (with known size a priori)] is filled later, e.g.:
https://github.com/ocornut/imgui/blob/master/misc/cpp/imgui_stdlib.cpp

Could you please consider to add resize() or even more - additionally with: capacity() and reserve() to mimic what the gang from std does... just to handle such callback-cases in a elegant way 😇

[ChemistAion]

For now, for this particular usage I have this thing:

struct InputTextCallback_Meta
{
    madX::Meta::String* string_;
    ImGuiInputTextCallback chain_callback_;
    void* chain_user_data_;
};

static int Meta_InputTextCallback(ImGuiInputTextCallbackData* data)
{
    InputTextCallback_Meta* user_data = (InputTextCallback_Meta*)data->UserData;
    if (data->EventFlag == ImGuiInputTextFlags_CallbackResize)
    {
        madX::Meta::String* string = user_data->string_;
        IM_ASSERT(data->Buf == string->data());
        
        string->move_from(madX::Meta::String("", data->BufTextLen < madX::Meta::StringShortLength ? madX::Meta::StringShortLength : data->BufTextLen));
        data->Buf = string->data();
    }
    else if (user_data->chain_callback_)
    {
        data->UserData = user_data->chain_user_data_;
        return user_data->chain_callback_(data);
    }
    return 0;
}

bool ImGui::InputTextWithHint(const char* label, const char* hint, madX::Meta::String* string, ImGuiInputTextFlags flags, ImGuiInputTextCallback callback, void* user_data)
{
    IM_ASSERT(string);
    IM_ASSERT((flags & ImGuiInputTextFlags_CallbackResize) == 0);
    flags |= ImGuiInputTextFlags_CallbackResize;
   
    InputTextCallback_Meta meta
    {
        string,
        callback,
        user_data
    };

    return InputTextWithHint
    (
        label,
        hint,
        string->data(),
        string->size() + 1,
        flags,
        Meta_InputTextCallback,
        &meta
    );
}

Works flawlessly, but it is not elegant, take a look on Meta_InputTextCallback(...) where I have a hack (forced heap storage) instead of nice .resize() - due to: #190

btw:

• madX::Meta::String is using cista::raw::string
• constexpr cista::generic_string<>::msize_t madX::Meta::StringShortLength = (cista::generic_string<>::short_length_limit + 1);
• all my default initialization are: Meta::String("", madX::Meta::StringShortLength) to make ImGui::InputTextWithHint(...) happy;

[ChemistAion]

I will check the impact here of what @khng300 did in the: #187