Support user-defined y_doc document type

[trungleduc]

Problem

The current y_doc handler of JupyterLab uses the file_type of the document to create the corresponding y_doc document, but JupyterLab and jupyter_server hard-coded the file types to file | notebook | directory so any new file type registered with jupyter_ydoc can not be used.

Proposed Solution

Add an optional parameter to the path property of IDocumentProviderFactory.IOptions to specify the type of y_doc
document.

[trungleduc]

cc @hbcarlos @davidbrochart

[davidbrochart]

Thanks @trungleduc, yes that is needed to support collaborative editing for new document types.

[trungleduc]

Fixed by jupyter-server/jupyter_server#895.