Modern concurrency support

[bbrk24]

No description provided.

[stackotter]

I've tried building a few of my local projects with these changes, and the main things I've bumped into are;

• view initialisers should be nonisolated where possible (to allow for things such as storing/producing Images from stored/computed properties)
• button actions should be just @MainActor (without @Sendable) if possible?

I'll try with a few more projects when I get the time to see if anything else comes up.

[bbrk24]

I can try to make Image's initializer nonisolated, but in my personal projects when I need to send an image around, I traffic in ImageFormats.Image<RGBA>, not SwiftCrossUI.Image.