Relates to https://github.com/Qiskit/documentation/issues/6, which is
about docs. This PR only adds Prettier to our support code. We might
want to extend Prettier to docs in a follow up.
Having a consistent format is convenient and saves us time so that
humans don't have to manually move things around.
We have a non-trivial amount of TypeScript tooling, so this is worth
adding imo.