experimental SVG support

Adds a feature flag "svgomg-enabled" that directs SVG exports at an experimental SVG export engine: https://github.com/adobe-research/svgObjectModelGenerator

This warning should be updated, since a.) it isn't an API any more, and b.) it's in generator-assets.

I would still have a warning there, and state that any breaking behavioral changes (or removal of the experimental feature) will only bump the "patch" number.