Issue Description:

I would like you consider adding support for @ViewBuilder to be used with properties, so that the memberwise initializer Swift generates for structs automatically gains that function builder.

As an example, if I wanted to implement a custom SwiftUI VStack right now, I might start with this:

struct CustomVStack<Content: View>: View {
    let content: () -> Content

    var body: some View {
        VStack {
            // custom stuff here
            content()
        }
    }
}

However, that doesn't support @ViewBuilder, and so this kind of code would not work:

CustomVStack {
    Text("Hello")
    Text("Hello")
}

To fix this, I need to define a custom initializer:

init(@ViewBuilder content: @escaping () -> Content) {
    self.content = content
}

In this simple example it isn't a massive problem, but often that initializer has to copy in lots of values – it does exactly what the memberwise initializer did now just with @ViewBuilder for the single content property.

Ideally I'd like to be able to write something along these lines:

struct CustomVStack<Content: View>: View {
    @ViewBuilder let content: () -> Content

    var body: some View {
        VStack {
            // do stuff
            content()
        }
    }
}

Thank you!