Support converting Vega specs to Vega-editor compatible URLs

[jonmmease]

Inspired by @NickCrews's idea in vega/altair#3188, vl-convert could potentially support converting Vega/Vega-Lite specifications to share URLs that are compatible with the Vega editor.

I haven't looked into the details, but @NickCrews mentioned that the editor is using the lz-string JavaScript library for URL-compatible compression. There is a Rust port of this library at https://docs.rs/lz-str/latest/lz_str/. If this library is compatible with the what the Vega editor needs, this might be a pretty easy and useful addition to vl-convert.

[NickCrews]

I don't really understand how all the vega parts fit together, if this is added here, would it be accessible from altair?

Specifically, vega editor uses the LZstring.decompressFromEncodedURIComponent method. The python port I mentioned in the above issue did not have this flavor. But the rust port you linked does! So it might just work out of the box! I would just try passing some json to the rust lib and see if it works as a 10 minute prototype

[jonmmease]

This turned out to be pretty straightforward, so I opened a PR at #104. Regardless of what else happens with the Vega editor, this is a nice feature to have in VlConvert.

I don't really understand how all the vega parts fit together, if this is added here, would it be accessible from altair?

vl-convert-python is an optional dependency of Altair that we already use for PNG and SVG image export. We would probably want to have Altair provide a convenience chart.to_url() method, but it will be usable with vl-convert directly like this:

import vl_convert as vlc
vlc.vegalite_to_url(chart.to_json())