Support configurable list of files for empty repository detection

### Is your feature request related to a problem?

Hi! My OSPO is interested in using this action to assist with our repository archival practices. We are building tools to identify repositories that are candidates for archival and plan to use this workflow to assist us in our work of reviewing contents of a repository.

Currently, the workflow is able to identify repos that are empty or README-only. In my organization, repositories are required to include additional standard files like CONTRIBUTING.md and SECURITY.md. We'd like the workflow to support a configurable list of files that, when present alone, still qualify a repo as empty.

### Describe the solution you'd like

We would like a new feature that supports a configurable list of files to be treated as "blank" alongside `README.md` such as `CONTRIBUTING.md`, `SECURITY.md` and other community/policy files.

### Describe alternatives you've considered

_No response_

### Additional context

I'm happy to work on this if it aligns with the project's direction. Just wanted to get your thoughts before diving in!

Beginne damit, die aktuelle Erkennung leerer oder nur README enthaltender Repositorys im Workflow sowie die Konfigurationseingaben der Action zu lokalisieren. Füge eine konfigurierbare Liste von Dateien hinzu, die als leer behandelt werden, und verifiziere anschließend, dass Repositorys, die nur README.md und die konfigurierten Dateien enthalten, weiterhin als leer erkannt werden.

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