Fix: const constructors for PatternEvaluators

[simon-the-shark]

I've added const keyword in two places.

This change is needed to allow us to use PatternEvaluators with code generation version of this plugin like this:

@RfControl(validators: [
      RequiredValidator(),
      PatternValidator(
        DefaultPatternEvaluator(
          r'^(https:\/\/)[a-zA-Z0-9\-\.]+\.[a-z]{2,3}(/\S*)?$',
        ),
      )
    ])
    String? url,