Support JupyterLab 2.0.0

[consideRatio]

Fixes #51.

After this PR is merged, future releases of this extension will now require JupyterLab 2, but if JupyterLab 1 is used an old versions of this package can still be used automatically instead.

[jasongrout]

I think it is fine to only support jlab 2 moving forward.

[consideRatio]

@jasongrout can you help me validate my understanding:

Assume we have a NPM package with two versions, version 0.1.0 with JupyterLab 1 dependencies, and version 0.2.0 with JupyterLab 2 dependencies. If I had JupyterLab 1 installed, and did jupyter labextension install <name>, it would install version 0.1 that was the latest version compatible with JupyterLab 1 dependencies?

[jasongrout]

it would install version 0.1 that was the latest version compatible with JupyterLab 1 dependencies?

Yes. JupyterLab is smart enough to go back through the published versions of a package and install the latest compatible version.

[granitDev]

As a user of Jupyter Labs, I would like to request this pull request be merged in as soon as possible. I would like to have this functionality back in version 2.

Thank you.

[jasongrout]

CC @wolfv

[wolfv]

Thanks everyone!

[consideRatio]

Thanks @wolfv!

This will be part of a release > 0.6.0: https://www.npmjs.com/package/jupyterlab-drawio