feat(anywidget): Hoist static assets (_esm, _css) to share among front-end widget instances

[manzt]

Towards #613

This PR is brings up some challenges for the ecosystem as a whole. I'm very tempted to go forward with #579. That would allow us to avoid the complexity of needing to work with ipywidgets (handling the comm ourself and messages), but currently all the ipywidgets container classes (e.g., Box, HBox, VBox) do an instanceof(widget, ipywidget.Widget) check. That means if we were to introduce a more minimal base class for anywidget.AnyWidget, they couldn't be used in those containers.

[changeset-bot]

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