Support configurable list of files for empty repository detection

まだ誰も着手していません。

### 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!

まず、workflow における現在の空のリポジトリまたは README のみのリポジトリの検出処理と、action の設定入力を特定します。空として扱うファイルの設定可能なリストを追加し、README.md と設定したファイルのみを含むリポジトリが引き続き空として検出されることを確認します。

索引モデルが issue の本文から書いたものです。