When not running in browser, emit svgs to disk

[longouyang]

No description provided.

[longouyang]

This is basically supported by 89f9280 but:

• I'm currently just logging the image SVG text to the console
• It's asynchronous because Vega is asynchronous (and simple approaches, like wrapping in a fibers library, don't work here because everything would need to be wrapped in a synchronous call, not just the rendering part of the webppl model)

[longouyang]

SVG now is emitted to disk.

Todo: write a mechanism for making vega rendering effectively synchronous (something like await for webppl), then we can close this.