Support file drop

[pscanf]

Support dropping an Excalideck file anywhere in the page to load it.

At the moment, the Excalidraw component tries to load the file, and gives the following error:

Notes:

• the "There are some unsaved changes" warning should still be shown, if necessary
• https://github.com/excalidraw/excalidraw/pull/3960 might be useful