Allow using a canonical dist file

[carlos-granados]

Most code quality/testing tools allow you to have a "canonical" (dist) config file which is stored in the repository and a local config file which replaces/extends the canonical file, allowing you to have a base configuration which is shared by all users and which can be overridden locally

This PR adds this capability to Rector. Rector will load a rector.php file if it exists but will also use a rector.dist.php file if the rector.php file does not exist

Adds two new end-to-end tests that check this new functionality

[TomasVotruba]

Let's ship this, thank you 👍

[dcs-61b676b2a0]

The documentation uses rector.php.

Should it default to rector.dist.php?
Should the generation question be changed? (Type N,n,NO,no not to generate, enter to generate rector.dist.php, other option is to type rector.php or any other filename.)

Perhaps, if something like it is available, a link referencing an explanation of the practice of the .dist interfix could be added to the documentation.