Use Vega resize handler

[domoritz]

The next version of Vega will support resize handlers: https://github.com/vega/vega-view/pull/18. These can be used to automatically resize the cell when the Vega view resizes.

[mbostock]

Thanks! We’ve also deployed a new version of Observable that uses ResizeObserver when available, so at least in browsers that support ResizeObserver, it should work today.