Add Github Workflow

[cjobeili]

With Travis CI not available, i've worked on the Github Actions integration for Silly

Testing for PHP:

• 7.0 (low-deps)
• 7.1
• 7.2
• 8.1
• 8.0
• 8.1 (low-deps)

Here you can see the results: https://github.com/cjobeili/silly/actions/runs/1813643403

Have a great day !

[mnapoli]

Beautiful!