Improve interoperability with native UI frameworks

[stackotter]

This PR introduces various APIs related to improving interoperability between SwiftCrossUI view code and native UI frameworks. This includes:

• Making it easier to make platform-specific modifications to the native widgets behind SwiftCrossUI views via the inspect family of modifiers
  • This allows people to work around missing SwiftCrossUI features and to access niche platform-specific APIs that we don't plan on wrapping
• Enabling the hosting of SwiftCrossUI views in native apps via the FooHostingView family of types
• Enabling the hosting of native views in SwiftCrossUI apps via the FooRepresentable family of types
  • (SwiftCrossUI already NSViewRepresentable, UIViewRepresentable and UIViewControllerRepresentable, but we still need to introduce similar APIs for the non-Apple backends)

So far I've completed the inspect family of view modifiers (which are housed in each backend), and introduced an AdvancedCustomizationExample app to showcase these new advanced customization APIs.