Adds appliesTo to verify if a given rule should be applied

[micheleorselli]

This is a tentative fix for #434. It introduces a new method, appliesTo, which is used to decide if a given rules should be applied to the class under examination, eg:

• final classes, enums, interfaces, trait, cannot be abstract so we exclude them when we check if a given class is abstract or not, because they cannot have that property by definition.

Caveat: with this fix a rule defined in that way

$rule = Rule::allClasses()
            ->that(new ResideInOneOfTheseNamespaces('App\Abstract'))
            ->should(new IsAbstract())
            ->because('we want to prefix abstract classes');

would miss any final class present in the namesapce App\Abstract. Is that what we want?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.71%. Comparing base (13d5528) to head (b61a388).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #454      +/-   ##
============================================
+ Coverage     93.62%   93.71%   +0.08%     
- Complexity      593      597       +4     
============================================
  Files            69       69              
  Lines          1601     1608       +7     
============================================
+ Hits           1499     1507       +8     
+ Misses          102      101       -1     

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[fain182]

would miss any final class present in the namespace App\Abstract. Is that what we want?

I don't think this would be the expected behaviour from our users....