initial rector dry-run setup

[ziming]

An initial setup of rector, this is different from my other PR.

My Other PR is more of a github action that run the downgrade rule on new PRs and suggest what code need to be changed to be compatible with PHP 7.3

This PR is more of something you and your team could run by yourself when you wanted to do some quick refactoring.

The configuration and comments are in rector.php. I included my most commonly used rules there with links to useful resources

To run it, try 1 of the following commands

composer rector-dry-run // i added an entry to composer.json scripts ./vendor/bin/rector --dry-run ./vendor/bin/rector process --dry-run

[promatik]

Let me add my note on this, I love this 🙌

Newest packages like language-switcher, calendar-operation and translation-manager already have PHPStan, we can probably add Rector there too.