transpile utilities so that V8 can keep up

[ijlyttle]

Right now, VL 5.1.0 works, but VL 5.1.1 does not - I think this is because:

• Vega-Lite has changed how they bundle their code: fix: generate es2015 output vega/vega-lite#7595
• V8, used for vw_to_svg() and vw_to_vega(), uses older version of V8 engine for Windows, which cannot run the new VL bundles: Is it possible to upgrade V8 for Windows? jeroen/V8#126

As pointed out in the V8 issue, a solution is to transpile what we need for vw_to_svg() and vw_to_vega() into ES5.

[ijlyttle]

This might be a bit drastic, but my current thought is to spin a set of functions into its own package, perhaps vegacompile or vegacommand:

• vw_to_vega()
• vw_to_svg()
• vw_write_svg()
• vw_write_png()
• vw_to_bitmap()

These all use Vega(-Lite) in a "node" context, rather than a "browser" context.

I think we would still use these functions from {vegawidget}, but have {vegacompile} as a "Suggests".

It might be useful to encapsulate all these headaches in their own package.

[ijlyttle]

this is obviated with the release of {V8} 4.0.0 🎉