Allow disabling the placeholder

[jeffrafter]

Summary

When at least two users are collaborating (for example using the WebrtcProvider) and a user selects all and deletes the document becomes empty. This triggers the PlaceholderExtension which creates a decoration node for the placeholder message. Typing (collaborative) becomes disabled in the editing user window until they hit enter. This is because the node position / selection is confused and YJS can't sync.

This allows you to disable the decorations in the placeholder. In general, I feel like this approach solves the issue (which is why I've opened this PR) but doesn't feel ergonomic. It would be better to remove the extension altogether in this case. An example of how that might be done is here: 9738b2c

Fixes #1993

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Latest commit: 0283db0

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[andrew-maltsev]

hey any updates? this is really crucial bugfix for collaborating on mobile and desktop