Switch to @prettier/sync for sync Nunjucks formatting

[colinrotherham]

Prettier recently switched to a Promise-based API which affects Nunjucks macro options format()

Prettier 3.0: Hello, ECMAScript Modules!
https://prettier.io/blog/2023/07/05/3.0.0.html

We'd need to replace Nunjucks for loops with asyncEach to give include blocks asynchronous support so we held the Review app on prettier@2 for now:

• Bump the lint group with 3 updates #3939
• Bump the lint group with 3 updates #3966

But this PR now takes the advice from the blog article above:

If you still need sync APIs, you can try @prettier/sync

With thanks to @fisker we can continue using tsc and our ESLint plugins:

• Check type declarations with JSDoc comments prettier/prettier-synchronized#10