Provide a flag parameter to allow limiting spyable availability

[dafurman]

See #41 for conversation leading up to this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a9062fb) 95.98% compared to head (353844a) 96.18%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #64      +/-   ##
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+ Coverage   95.98%   96.18%   +0.19%     
==========================================
  Files          15       15              
  Lines         573      603      +30     
==========================================
+ Hits          550      580      +30     
  Misses         23       23              

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[dafurman]

@Matejkob

From your suggestion here, I covered

Updating the README
Providing detailed documentation, with an example of a custom-defined flag as well as the DEBUG one

I'm happy to do it, but I'm not sure what you meant about refreshing the examples package, as it seems empty anyways to me right now. I'm not too familiar with DocC, in case you could give some direction here.

[Matejkob]

From your suggestion here, I covered

Updating the README
Providing detailed documentation, with an example of a custom-defined flag as well as the DEBUG one

I'm happy to do it, but I'm not sure what you meant about refreshing the examples package, as it seems empty anyways to me right now. I'm not too familiar with DocC, in case you could give some direction here.

Regarding the Examples package, I was referring to the one located at https://github.com/Matejkob/swift-spyable/tree/main/Examples. This is a separate Swift Package that contains practical examples demonstrating how to use the macro in real-world projects.

It would be great if you could refresh this Examples package by ensuring that it's up-to-date and accurately reflects the current capabilities and usage of our macro.

[dafurman]

Unfortunately due to the limitation of not being able to define preprocessor flags on packages, I don't really think there's an effective way to demo this without a full-fledged .xcodeproj. Not sure if it's worth the trouble to restructure the example just for this, when it's also documented, but let me know what you think. This also raises another limitation of this flag - it can't be used within packages.

[Matejkob]

Once again thanks a lot for your contribution! 🚀