Add a new option to the `bump-version` script to list files it updates

[mcdonnnj]

🗣 Description

This pull request adds a --list-files option to list out any files the bump-version script updates when changing versions. This is in addition to updating the script to store any files it updates in an array.

💭 Motivation and context

The array itself is a good improvement, but the new option is in support of https://github.com/cisagov/gh-skeleton/issues/31.

🧪 Testing

Automated tests pass. I verified the new option works as expected:

$ ./bump-version --list-files
config/version.txt
README.md

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] All new and existing tests pass.