[DO NOT MERGE] experiment: add @_spi attribute as an experiment

[ajpallares]

Experiment of @_spi attribute for future internal APIs (see https://swiftpackageindex.com/swiftlang/swift-syntax/600.0.1/documentation/swiftsyntax/spi)

[ajpallares]

@RCGitBot please test

[aboedo]

@ajpallares I'll add the pr:other label so Danger doesn't complain.

But... installation tests passed! 🎉

Although... should they have? LOL like, isn't it expected that things that use the Purchases.shared.appUserID property would have failed to compile? Am I missing something?

[ajpallares]

@aboedo well, it turns out I was too conservative when adding the @_spi attribute and added to a property that is also exposed via the PurchasesType protocol 🤦‍♂️
I'll be more aggressive now and test it in the configure method 😄
At least we now know that adding some @_spis does not break anything 😅

[ajpallares]

@RCGitBot please test

[aboedo]

tests fail now, uhh yay I guess lol 🎉

[ajpallares]

I think we can close this PR as it already served its purpose. @RevenueCat/coresdk is there anything else that is worth testing for this @_spi attribute?

[tonidero]

Hmm I think that covers everything 👍