feat: support custom widget button rendering

[AhyoungRyu]

Added renderWidgetToggleButton prop to enable custom widget toggle button rendering. The usage would be like

<ChatAiWidget
   renderWidgetToggleButton={(props) => (
     <div className="chatbot-button-area">
       <button
         onClick={props.onClick}
         className={`chatbot-icon ${props.isOpen ? 'on' : 'off'}`}
       >
          <span className="chatbot-icon-content">
           {props.isOpen ? 'CLOSE' : 'OPEN'}
          </span>
       </button>
      </div>
    )}
/>

[AhyoungRyu]

Let me update the README in the following PR.