Implement cleanup strategy for closePortal in useFlatfile hook

[coderabbitai]

There's a TODO comment in the hook within suggesting the need for a cleanup strategy, specifically for removing the iFrame/listener from the DOM. This is important to prevent potential memory leaks or unintended behavior.

Context: The TODO was highlighted during a review of PR #82, specifically in the discussion #82 (comment).

The issue involves deciding on and implementing a strategy to ensure that resources are properly cleaned up when the function is called. This may include removing iFrames and detaching event listeners that were added during the portal's operation.

This task is crucial for maintaining the integrity and performance of the application, preventing memory leaks, and ensuring that event listeners do not persist beyond their intended lifecycle.

Requested by: @bangarang