`auto-changelog init` does not produce a Prettier-compliant changelog

## Problem

- When creating a new changelog via `auto-changelog init` in a project that validates the changelog via `auto-changelog validate --prettier`, the validation step will fail, forcing the user to correct the changelog. This is annoying.

## Suggested solutions

- The `toString` method on Changelog should produce a Prettier-compliant changelog.
- Prettier should always run after initializing, updating, or validating a changelog, and the `--prettier` option should be renamed to `--no-prettier` (or perhaps `--disable-prettier`).

Start with the Changelog `toString` method and the `auto-changelog init` command path. Reproduce initialization in a project using `auto-changelog validate --prettier`, then inspect how formatting is applied during initialization, updating, and validation. Done means a newly initialized changelog passes the existing Prettier validation without manual correction.

Written by the indexing model from the issue text.