Declare `createSynchronizedPrettier` as only named export

[colinrotherham]

As discussed in https://github.com/alphagov/govuk-frontend/pull/3971#discussion_r1265398678 @prettier/sync doesn't support named exports:

import { format } from '@prettier/sync'

This PR updates the type declarations to show that

I've also updated the tsconfig.json file so local test.js imports are checked too