Make scripts more DRY

[remcotolsma]

We have same code in place in:

• https://github.com/pronamic/action-wordpress-plugin-release/blob/5af50dc178b3d9cf7e86867115d827ec0bea6cf2/release.php
• https://github.com/pronamic/action-wordpress-plugin-readme-update/blob/ed9e35d381e5441c13f2657369829fb9765073ad/deploy.php

I noticed that Composer is by default available:
https://github.com/actions/runner-images/blob/main/images/ubuntu/Ubuntu2204-Readme.md#php-tools

We could make a helper library for the CLI, maybe a bit like:
https://github.com/actions/toolkit/tree/6c4e082c181a51609197e536ef5255a0c9baeef7/packages/core

https://github.com/splitbrain/php-cli