Set up Prettier formatting configuration

[erickzhao]

This repo contains a .prettierrc.json configuration file but no devDependencies referencing Prettier. We should either get rid of the Prettier config or set up ESLint to interop with Prettier as per https://prettier.io/docs/en/integrating-with-linters.html.

https://github.com/electron/windows-sign/blob/main/.prettierrc.json