🚀 Feature: Allow filtering on PR author

### Bug Report Checklist

- [X] I have pulled the latest `main` branch of the repository.
- [X] I have [searched for related issues](https://github.com/JoshuaKGoldberg/prune-github-notifications/issues?q=is%3Aissue) and found none that matched my issue.

### Overview

Right now, `prune-github-notifications` allows filtering on `reason` and `title`. But sometimes an individual will send a PR that just so happens to look like a bot's PR. It'd be nice to be able to filter to just the PRs sent by the bot. A couple of example use cases:

* `author: [/^dependabot[bot]$/]`: would work for many straightforward repos
* `author: [/^SomeUser$/, /[bot]$/]`: would ignore one user and all bots

Let's add in an `--author` CLI option & `author` Node.js option!

### Additional Info

_No response_