AnyWidget

[keller-mark]

Fixes #95

The backend-rendering widget is quite slow. Perhaps only the WASM-rendered widget is feasible.

TODO:

• Test the Zarr data transfer message passing in the WASM-based widget
• Should the WASM widget be a separate package entirely (rather than an extras_require), since it does not depend on the non-wasm-compiled Rust?
• Bundle the Pluot JS with the widget, rather than using a CDN/unpkg for JS deps in the widget

[keller-mark]

TODO:

• adopt changes from Convert async custom message handlers to sync vitessce/vitessce-python#506