Feature: allow `include` to allowlist certain packages

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

Rather than just an `exclude` list to prevent certain packages from automerging, could there be an `include` option, to allow _only_ those packages to automerge?

### Motivation

Right now if we want to limit automerging to only certain packages we trust, we would need to update the `exclude` list every time a developer adds a new package, which is likely to be forgotten. Allowing us to only mark certain packages as allowed via an `include` list would solve this problem, and feels in keeping with the rest of the action.

### Example

```yaml
steps:
- uses: fastify/github-action-merge-dependabot@v3
with:
include: 'vitest,eslint'
```