Png export

[fxgenstudio]

Add png image support.

[jaime-olivares]

Still looking at this. Cannot find a clean way to do it.
The VSCode extension API doesn't provide access to the DOM, so it cannot be done by using DOM objects.
Don't really want to use a dependency like PhantomJS or ImageMagick, as there is no certainty to work well in all installations.

[jaime-olivares]

This won't happen anytime soon. there are lots of technical barriers for doing this smoothly in pure javascript. I am reluctant of using any kind of 3rd party binary as it will trigger a lot of installation issues in multiple platforms.