[Collaboration] Make yjs provider composable

[Bojie Yang]

Is your feature request related to a problem? Please describe.

Currently the collaboration structure only support one yjs provider. But the yjs providers are composable.

For example, I may want to compose y-indexeddb and y-partykit/provider to support offline editing and automatic sync to the server when it is online. yjs support this feature in it's document, but it can't used in blocknote.

Describe the solution you'd like

Allow set multi providers in the collaboration structure or other way to make the provider composable .

[Yousef]

I'm pretty sure this is supported; just add multiple providers to the document