SquiggleOutputViewer refactorings and empty results

[berekuk]

This PR is mostly refactorings (it splits out ViewerBody and ViewerMenu from SquiggleOutputViewer).

I wanted to add some debugging views to the viewer, but got distracted by cleanups.

It includes two small user-facing changes:

1. Empty Squiggle programs are now valid; previously, we checked for code === "" on the components side, but an empty program with a comment still failed; now empty programs are allowed in Peggy grammar
2. VoidWidget; now empty result page doesn't look broken:

Another internal change is that SqDict now exposes isEmpty() and size(). I couldn't resist removing getResultVariables and so on, they looked too trivial and copy-pasted to me; so I replaced them with direct lookups on SqOutput.

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Latest commit: bf69e56

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[OAGr]

I think this looks good, will have a better idea when I start using it. Want to merge soon to minimize conflicts.