fix: add Async Image

[hyeonghwan]

Issue

An error occurred. in CDMarkdownImage class -> "let data = try? Data(contentsOf: url)"
Synchronous URL loading of https://upload.wikimedia.org/wikipedia/commons/thumb/1/14/Gatto_europeo4.jpg/250px-Gatto_europeo4.jpg should not occur on this application's main thread as it may lead to UI unresponsiveness. Please switch to an asynchronous networking API such as URLSession.

Goals ⚽

My goal is to asynchronously fetch an image and reflect it in the view to resolve this error.

Implementation Details 🚧

I added the AsyncTextAttachment class to fetch the image asynchronously, and I wrote an extension for NSLayoutManager to track the size of the attachment. When the startAsyncImageDownload() function is executed, a function is called to apply the layout. And when the task is completed, it calls the delegate to notify that it has finished.

Testing Details 🔍

I added "#if os(iOS)" because I tested it only on iOS.
It works well on iOS.