Generic Vega-Lite Client

[domoritz]

https://github.com/uwdata/mosaic/tree/main/packages/vega-example has a hard-coded example showing that Vega and Vega-Lite work with Mosaic. In this project, you create a more general client that takes a Vega-Lite specification and automatically extracts aggregates and binning so they can be handled by Mosaic.