feat: added modal.onMounted handler

[bang9]

• Added modal.onMounted handler to global eventHandlers.
  This event handler is triggered within the useEffect of the Modal component at mounting time.

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Example usage

const App = () => {
  return (
    <SendbirdProvider
      eventHandlers={{
        modal: {
          onMounted({ id, close }) {
            // Push the state to prevent the browser's back button press
            window.history.pushState({ id }, '');

            // Close the modal if the back button is pressed
            const onPopState = (e: PopStateEvent) => {
              e.preventDefault(); // Prevents the default back action
              close(); // Closes the modal
            };
            window.addEventListener('popstate', onPopState);

            // Cleanup function
            return () => {
              // Clear the current state when the modal is closed without using the back button
              if (window.history.state?.id === id) window.history.back();
              window.removeEventListener('popstate', onPopState);
            };
          }
        }
      }}
    />
  );
};

ticket: CLNP-3057

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[HoonBaek]

LGTM