Better typing for outputs

[krassowski]

Problem

Currently outputs are not typed. This makes it hard to track changes in public API or to work with this package in general.

Previously at least the changes were typed as nbformat.IOutput but this changed in https://github.com/jupyter-server/jupyter_ydoc/pull/241 and now they are just Map<any>:

https://github.com/jupyter-server/jupyter_ydoc/blob/546a99442b83482ecab91f953eaa35337765fddf/javascript/src/api.ts#L746-L748

On the Python side the schema defines them as just an array, without a type:

https://github.com/jupyter-server/jupyter_ydoc/blob/546a99442b83482ecab91f953eaa35337765fddf/jupyter_ydoc/ynotebook.py#L35-L46