Create Workflow to Validate db/bans.cfg on PRs and Pushes to Main

[sonikro]

Implement a GitHub Actions workflow that performs the following on every pull request and push to the main branch:

• Validates that the file db/bans.cfg is a valid CSV file (checks for proper CSV structure and format).
• Validates that each entry in the file contains a SteamID in the correct Steam3 ID format (e.g., U:1:123456789).

This workflow will help ensure that bans are correctly formatted and prevent invalid entries from being merged.