framework/view: experiment to extract out svelte into a swappable "viewer"

[matthewmueller]

This PR is an experiment to support swapping out the view renderers. Currently we have just one Svelte "viewer", but it'd be nice to be able to define your own viewers. This would allow for other viewers like GoHTML, HTML and React.

I haven't decided if this interface should be exposed as a generator yet, but at the very least the internal interfaces will be capable of supporting other types of renderers. If it does make sense to expose it, what should we call this? Viewers? Renderers? Templates? Something else?