feat: add PHAR generation on tag

[owenvoke]

This adds a Box PHAR workflow to generate a PHAR for Drift when a new tag is pushed.

Check out https://github.com/owenvoke/drift/actions/runs/208504306 for an example.

What this does:

• Creates PHAR
• Adds it as an "artifact" which lets you download the generated PHAR through GitHub's UI
• Creates a release on GitHub and adds the PHAR as an asset

Issues

• Probably need to get PHAR scoping working before merging, I had issues locally (see CI workflow here).
  This should now be working...
• The PHAR does not actually work (🤦🏻) as the RectorRunner executes the Rector binary, which is inside the PHAR so can't be run...

Closes #21

[olivernybroe]

Awesome, hope you solve the issues so the phar works 👍