Consider running prettier separately

[amannn]

We currently run prettier as part of ESLint via eslint-plugin-prettier. The intention of this was to easily integrate Prettier with our toolchain, but this approach is no longer recommended. We might want to run prettier separately from ESLint.

An added benefit is that by requiring a .prettierrc file, we'd apply our formatting rules to other files that are not linted by ESLint (e.g. markdown, styles, …).

See also: https://prettier.io/docs/en/configuration.html#sharing-configurations