Add ShapeStyle support to @ConentBuilder, allowing for more dynamic custom ShapeStyle implementations

AdaptiveStyle.swift

/// A ShapeStyle that adapts dynamically based on the environment's ColorScheme
///
/// This exists as a basic example of how to apply a @ContentBuilder to ShapeStyles
struct AdaptiveStyle<Light: ShapeStyle, Dark: ShapeStyle>: ShapeStyle {
    let light: Light
    let dark: Dark

    @ContentBuilder
    func resolve(in environment: EnvironmentValues) -> some ShapeStyle {
        if environment.colorScheme == .light {
            light
        } else {
            dark
        }
    }
}

extension ShapeStyle {
    static func adaptive<Dark: ShapeStyle, Light: ShapeStyle>(
        light: Light,
        dark: Dark
    ) -> Self where Self == AdaptiveStyle<Light, Dark> {
        .init(light: light, dark: dark)
    }
}

ContentBuilder+ShapeStyle.swift

// Augment ContentBuilder with basic functionaly to support ShapeStyle as Content
extension ContentBuilder {
    // SwiftUI's built-in _ConditionalContent is explicitly non-Sendable, and ShapeStyle is
    // Sendable, so we need to supply our own supporting type for conditionals
    enum _ConditionalShapeStyle<True: ShapeStyle, False: ShapeStyle>: ShapeStyle {
        case trueContent(True)
        case falseContent(False)

        func _apply(to shape: inout _ShapeStyle_Shape) {
            switch self {
            case .trueContent(let content):
                content._apply(to: &shape)
            case .falseContent(let content):
                content._apply(to: &shape)
            }
        }
    }

    static func buildOptional<S: ShapeStyle>(
        _ component: S?
    ) -> _ConditionalShapeStyle<S, Color> {
        if let component {
            return .trueContent(component)
        } else {
            return .falseContent(.clear)
        }
    }

    static func buildEither<TrueContent: ShapeStyle, FalseContent: ShapeStyle>(
        first: TrueContent
    ) -> _ConditionalShapeStyle<TrueContent, FalseContent> {
        .trueContent(first)
    }

    static func buildEither<TrueContent: ShapeStyle, FalseContent: ShapeStyle>(
        second: FalseContent
    ) -> _ConditionalShapeStyle<TrueContent, FalseContent> {
        .falseContent(second)
    }
}