Support getClipboardFiles() for Web

[EchoEllet]

Currently getClipboardFiles() is unsupported for the web, I haven't taken enough time to see how it will work on the web or if it's a common and standard feature on the web. It's common to drag and drop files but I'm uncertain if I have copied files from an application or in the file manager on a desktop and then pasted them on a web application. And which restrictions are imposed on browsers like Firefox and Safari that should be handled?

Once fixing or closing this issue, update this related TODO on flutter-quill repo.

Part of singerdmx/flutter-quill#2230