Add "Custom fixers" option

[devatreides]

When using a linter tool, sometimes the project requires a custom rule to meet specific needs. To cover such cases, PHP-CS-Fixer provides a custom fixer feature where you can create your own Fixer and register it in Finder. Today, when using Pint, it's not possible to register a custom rule, so when the project has this type of need, the developer ends up using PHP-CS-Fixer instead of Pint.

This PR adds a new option to the pint.json configuration file: custom-fixers. This option allows developers to list an array of custom fixer files they want to register and then declare the custom rule in the rules option.

Here is an example of a pint.json file with this option set:

  "custom-fixers": [
      "path/to/CustomFixer.php"
  ],
  "rules": {
      "custom-rule": true,
      ...
  }

Important to mention that the path is not absolute but relative to the root of the project.

[taylorotwell]

IMO you should use php-cs-fixer instead of Pint in these cases. The entire purpose of Pint is to be extremely opinionated with minimal configuration.