Add Railway deployment support

[infomiho]

Adds Railway as a deployment platform option alongside existing Fly.io support. Users can now choose between platforms using the platform input.

Changes

Core Functionality

• ✅ Add platform input (required): Choose 'fly' or 'railway'
• ✅ Add Railway-specific inputs: railway-token, project-name, railway-workspace
• ✅ Conditional CLI installation (Flyctl for Fly, Railway CLI for Railway)
• ✅ Conditional deployment logic for both platforms
• ✅ Improved bash variable handling

Documentation

• 📝 README updated with TODO comments for final documentation
• 📝 Sections planned for: Fly setup, Railway setup, platform comparison, breaking changes

Example Usage

Fly.io (existing users must add platform: fly)

- uses: wasp-lang/deploy-action@main
  with:
    platform: fly
    fly-token: ${{ secrets.FLY_TOKEN }}

Railway (new)

- uses: wasp-lang/deploy-action@main
  with:
    platform: railway
    railway-token: ${{ secrets.RAILWAY_TOKEN }}
    project-name: my-app

Breaking Changes

⚠️ Existing users must update their workflows to add platform: fly

TODO

• Complete README documentation (replace TODO comments with actual content)
• Test with real Fly.io deployment
• Test with real Railway deployment