Fixes multiple bugs

[hbcarlos]

Necessary to fix tests in JupyterLab.

Code changes:

• Dependency process: When linking @jupyter/ydoc to jupyterlab in dev mode, JupyterLab complains about ydoc missing this dependency. I added it in devDependencies so we only install it in dev mode.
• I changed the argument of the static method YNotebook.create to be able to initialize the notebook in a generic way. For example, JupyterLab needs to initialize the model with nbformat, nbformat_minor, and metadata but other frontends might need other attributes.
• Fixed the imports
• Fixed the callbacks for metadata changed in YCell and YNotebook. Both oldValue and newValue might be undefined.

[hbcarlos]

Regarding the tests, I moved them to different files. The only changes are that I added YNotebook/factory as you requested. In addition, I added a few tests for the YFile.