Use trailing commas

[vatro]

I got really annoyed by import-diffs including preceding lines while refactoring SvelthreeInteraction, so let's just do it.
But I won't do this now, in the middle of refactor. I'll do it after merging these two branches:

• https://github.com/vatro/svelthree/tree/204-implement-wheelevent
• https://github.com/vatro/svelthree/tree/204-refactor-svelthreeinteraction

The only thing that's really annoying me while refactoring SvelthreeInteraction are the import blocks, but that's also just because I'm moving a lot of logic out of the component, removing and adding imports.