Run prettier to catch *.hbs files (and a few misc json/vue files)

[rsek]

depends on #466 . prettier does throw some errors about not supporting some *.hbs features (decorators, "../" syntax in Glimmer), but to be frank i don't know enough about handlebars to resolve those yet. i'll pick over the prettier HBS docs to see if i can puzzle it out, but this PR is probably(?) fine?

[ben]

Rebase please? It'll be a lot easier to review if the diff is more minimal.