Add labextension develop cli

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Fixes #29

• Adds packages with minimal metadata (will form the basis of a cookiecutter that just has the shim module).
• Surfaces a disabled_labextensions trait to take the place of the previous page_config version. We will use this instead of the way server_extensions are enabled/disabled since these can be specific plugins within extensions.
• Adds labextension develop command
• Includes dynamic extensions in list
• Adds support for uninstalling dynamic extensions

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A thought: jupyter labextension link could be reclaimed for this, and use the symlink feature from nbextensions. Our watch command would be simpler, and link didn't work all that well. We'd be removing support for linkedPackages, but evidence has shown that it was broken anyway.

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@jasongrout I'm going to build out develop for now, but what do you think about reclaiming link?

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$ python labextensions.py uninstall @jupyterlab/example-federated-md
Removing: /Users/stslve/miniconda/envs/module-federation/share/jupyter

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$ python labextensions.py disable @jupyterlab/example-federated-md
$ python labextensions.py list
JupyterLab v3.0.0a4
/Users/stslve/miniconda/envs/module-federation/share/jupyter/labextensions
        @jupyterlab/example-federated-md v2.1.0 disabled OK
$ python labextensions.py enable @jupyterlab/example-federated-md
$ python labextensions.py list
JupyterLab v3.0.0a4
/Users/stslve/miniconda/envs/module-federation/share/jupyter/labextensions
        @jupyterlab/example-federated-md v2.1.0  enabled  OK