`ICollaborativeContentProvider` should likely inherit from `IContentProvider`

[krassowski]

That seems to have gotten us past that error, but now:

Property 'get' does not exist on type 'ICollaborativeContentProvider'.

🤔

Hmm the implementation actually does have a get() method. This is due to the interface of ICollaborativeContentProvider not inheriting from IContentProvider. I think we may want to expose it if this is desirable.

Originally posted by @krassowski in geojupyter/jupytergis#848 (comment)

jupyter-collaboration/packages/collaborative-drive/src/tokens.ts

Line 25 in 67453e0

export interface ICollaborativeContentProvider {