fix: Loading indicator when dropping/pasting files

[matthewlipski]

This PR adds a loading prop to the default file blocks, which causes them to render a Loading... indicator when true. Unfortunately, the delay is happening in editor.uploadFile, which is not really related to the block itself. So that means we don't have an existing way of checking if a file is being uploaded while the block is rendered.

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[YousefED]

I don't think a loading prop is suitable for this. For example:

• It will mean the "loading" value will be part of the (stored) document and stored in the db
• it will conflict with history (undo / redo, etc)

we'll need to brainstorm a different architecture

[matthewlipski]

Do you have anything in mind? I was thinking we could expose an editor.fileUploadInProgress property which stores undefined or a block ID depending on if a file is currently being uploaded, and which file block it's being uploaded for. Then ig we can check that in the render function and avoid setting it in the state. Does this sound like a reasonable idea?