Add vue-widget - work in progress

[oscar6echo]

I have tried and failed to create a vue-widget similar in principle to the react-widget.
I stumble on low level typescript/vdom and don't understand many aspects of jlab building.

This is related to issue #201, in particular inspired by this old repo: jupyterlab_vue

If a JLab expert could have a look at it and give me hints about what glue code is missing, I'd could maybe try to make it work.

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