Inlined build for perspective-jupyterlab, improve PerspectiveWidget

[sc1f]

This PR changes the perspective-jupyterlab build to inline its external dependencies on the main Perspective modules, and makes some long-overdue improvements to PerspectiveWidget in order to enhance the user experience in Jupyterlab and Voila. Specifically:

• Fixes Perspective 0.7.0 and above does not work in Voila #1454, which was due to Voila not being able to access perspective-jupyterlab's external dependencies on the main Perspective modules. While the asset produced (10.6MB as loaded into a Jupyterlab production build) is not ideal, it is guaranteed to work in both Jupyterlab and Voila without additional hard-to-maintain/brittle build cruft.
• Fixes long-running bug where PerspectiveWidget had a height of 0 when rendered inside Voila. The height now defaults to 520px, the same as in Jupyterlab.
• The <perspective-viewer> config panel now shows by default on all widgets when rendered.
• worker.terminate() and viewer.delete() is now called whenever the widget output is disposed of, which should make memory usage in Jupyterlab more efficient.
• Fixes Format height for PerspectiveWidget #1462: PerspectiveWidget now inherits from ipywidgets.DOMWidget instead of ipywidgets.Widget, which means that layout should work out of the box:

• plugin_config can now be passed in from Python, which should allow for much finer-grained control over styling of the datagrid and chart plugins. However, because plugin_config cannot be read synchronously from the Javascript front-end (as it is not part of the attribute API and there is no async get in Javascript), there is currently no way to read the current plugin_config from the widget unless it has been set by the user already. However, this is still very useful for developers to "pre-set" styles on PerspectiveWidget:

• Modified WebSocketServer to strip out semver-like ranges from requests. This is helpful when debugging Voila locally, as I can just set the Voila CDN to localhost:8080/node_modules/ and all the CDN requests Voila makes will automatically be resolved against my local build.

[timkpaine]

Should this include jupyter widgets external?

[dhirschfeld]

Super keen to get this released so I can get my hands on the voila fixes! 🚀

[texodus]

Thanks for the PR! A couple post-review notes:

• There is a non-zero chance this change does not actually fix Voila, as we can't fully test the interaction with CDNs and Perspective's multiple-asset build has had CDN issues in the past.
• I still do not fully understand much of JupyterLab/Voila packaging, e.g. why @finos/perspective needs to be inlined, or conversely why @juypter-widgets does not. This combined with minimal testing (though much improved on the JupyterLab side since Add Jupyterlab tests to CI #1460) is a recipe for future regressions here.

[dhirschfeld]

Unfortunately I can't help with the development or packaging but I'm happy to test the JupyterLab/Voila combo on our deployment and report back any issues...

[sc1f]

@dhirschfeld @texodus

I have just tested the 0.10.0 release in a fresh virtualenv with perspective-python installed from wheel, @finos/perspective-jupyterlab installed from NPM, and everything works. Voila falls back to JsDelivr, and loads the plugin and all externals correctly with or without --enable_nbextensions=True set when running Voila.