Feature: auto-detect Prettier as a post-processor in CLI

### 🚀 Feature Request

In the `npx typestat` getting started CLI that generates a `typestat.json`, if a Prettier configuration is detected, it should offer to add it as a post-processor.

#### Existing Behavior

Mutators don't generally put effort into respecting Prettier settings. As a result, auto-fixed code often would not pass Prettier.

#### Change Proposal

> "Hey, would you like to have Prettier run on mutated files?"