fix: rules method overrides already defined rules

[maicol07]

Example: Calling required() before rules() doesn't add required to rules

Related to #567

[what-the-diff]

PR Summary

• Enhancement in Method for Assigning Field Rules
  A refinement was made in the rules() method found in the Field.php file. The introduction of += operator aids in supplementing the pre-existing set of rules of the field with more, rather than outright replacing them. This allows for more dynamic and flexible rule management.

[binaryk]

Thank you!