Pass Vega specification to Vega Editor for preview and edits

This is a feature request to improve online version of SandDance integration with online Vega Editor.

Synopsis:

Currently, SandDance online version has an option to Open Vega Editor for a chart in Chart Settings -> Tools section, Show Vega spec dialog. However, that dialog only links to online Vega Editor and doesn't pass the generated chart Vega spec for edits and preview in that tool.

Feature request:

Create compact Vega spec json string and Url encode generated Vega json for your chart using json-stringify-pretty-compact and lz-string libraries to pass it as a parameter to Vega Editor.

You can glean some info on how Vega Editor encodes vega specs in this vega-unfurl dev to post: https://dev.to/tarasnovak/vega-unfurl-take-2-5fnb

The general patern is: https://vega.github.io/editor/#/url/vega/<lz-vega-json-compact-compressed-string>

@danmarshall Let me know if you have questions about this integration proposal.

Start at the online SandDance Chart Settings → Tools section and the existing Show Vega spec dialog link. Trace how the generated chart Vega specification reaches that dialog, then verify that the Vega Editor URL contains the compact, encoded specification and opens it for preview and edits.