Introduce Fallback Mode to automatically fallback to native SwiftUI SDK

[Maschina]

Issue #68

Describe your changes

• Proposing again the idea to automatically fallback to native SwiftUI SDK
• I am currently testing it only for ContentUnavailableView

How do you like generally the implementation? I wanted to get feedback before proceeding with other shared components.

[shaps80]

I'm really not keen on this idea. I also don't really see a need for this. The point of the library is to generate compile time warnings or errors when the dev updates, and then go through the project like a "checklist" - this is an important step in my experience.

For me there are 2 key issues with this approach:

1. Where fallbacks have been enabled, it could lead to silently swallowing issues - this is an unhelpful developer experience

2. This would result in many conditions in View body code. This is can cause significant performance degradation in SwiftUI and while I don't claim to have a highly optimised back port of all implementations, I certainly don't want to include something that I know for a fact has a measurable impact.

As always I really do appreciate the thought and contribution here. I always love to see what others bring to the table, but in this case I remain unconvinced that this is a good idea and has enough positives to outweigh the negatives.

I'm open to further discussion but at this stage I don't feel this is something worth spending time on.