add a jlpm "bootstrap" entrypoint that can run during eg pip install of jupyterlab

[telamonian]

The fact that we can't reliably execute jlpm during the javascript install steps of eg a pip development install of jupyterlab causes some pain currently. It will cause more pain and real problems when we migrate jupyterlab to yarn v2.x+ (see: jupyterlab/jupyterlab#10929). If we include a "bootstrap" version of the jlpm entrypoint that installs as part of jupyterlab_packaging this problem goes away.

jupyterlab_packaging will only ever get installed transiently in a pip (or whatever py build tool) build env, and said install will only exist as long as the build is running. So in theory, at the end of a jupyterlab build the jlpm provided by jupyterlab_packaging will just gracefully evaporate and be replace by the jlpm provided by jupyterlab itself. We'll see how that works out in practice

[telamonian]

pinging @jtpio @blink1073. If anyone can give me explicit instructions on how to test these changes as part of eg pip install -e ./jupyterlab, that would be super helpful

nm, I figured it out