Enforce that one of the changelog entries in a new release is marked as breaking if bumping to a major version

## Problem

Some teams bump a package by a major but don't adequately call out breaking changes in the changelog when they do so. It is convention to mark these changes using a `**BREAKING:**` prefix. At least one of these entries should be present.

## Acceptance Criteria

- A release PR cannot be merged if a workspace package is being bumped by a major, but there is no entry present in the changelog for the new version with a `**BREAKING:**` prefix.
- Bonus points if we write this in such a way that when https://github.com/MetaMask/core/issues/5553 is implemented, the user will receive a bot comment that tells the user the error and shows them how to fix it.

## Related issues

- https://github.com/MetaMask/core/issues/4119
- https://github.com/MetaMask/auto-changelog/issues/109
- https://github.com/MetaMask/core/issues/5553

Start by tracing how release PRs validate workspace package version bumps and how changelog entries for new versions are read. Review related issues 4119, 5553, and auto-changelog issue 109 for context. Done means a major bump cannot merge without a `**BREAKING:**` entry, with an actionable error suitable for a future bot comment.

Written by the indexing model from the issue text.