Package prettier with Svelte support and add it to treefmt-nix to format
the dashboard. This change is brutal, I spent a long time trying to get
it nicer but it doesn't seem there's a good way to make this minimal.
Sorry for the noise!
This will make it easier for new contributors to get the formatting
right first time.
Also removes the `.prettierrc` because it turns out treefmt-nix was
ignoring it.
Test plan:
- CI
