fix!: update for compatibility with the new async APIs in Prettier 3

[steveluscher]

This PR:

• pins the peerDependencies version of Prettier to >=3
• updates the callsites of the Prettier API to reflect the fact that they're now async

This is a breaking version that will require a major version bump.

Fixes #586.

[steveluscher]

I forgot that I'd done this, discovered that our Prettier tests weren't doing anything, debugged the root cause, and then rediscovered this.

Any chance you'd like to crush out a release?