Format snap.manifest.json with Prettier

[Mrtenz]

Right now we take the output from JSON.stringify and put it in the snap.manifest.json. This results in issues as described in MetaMask/test-snaps#71. As a solution, we could format the manifests with Prettier before writing to the disk.

Prettier has a nice API, so it's essentially just a case of calling prettier.format(config, options). For the options, we could default to the user's .prettierrc, and use Prettier's defaults otherwise.