We were sticking to VScode's built-in formatter, since I thought `primer-app.code-workspace` was the only "jsonc" file in the project. But it turns out that the `tsconfig.json` files also allow comments, and we use Prettier on those.

As was configured in the previous commit.

Prettier handles this for us on all source files anyway, and for other files we needn't care (there is currently trailing whitespace in the README, at least).