feat: Add parent activated modifier

[ajinumoto]

Goals ⚽

• Add parent-level skeleton activation.
• Allow child views to call .skeleton() without passing with.
• Keep explicit child loading state as an override when .skeleton(with:) is provided.

Implementation Details 🚧

• Added .skeletonActive(_:) as a parent view modifier using SwiftUI environment.
• Updated .skeleton(with:) so with is optional and defaults to nil.
• When with is nil, skeleton state is resolved from the parent environment.
• When with is true or false, the child uses that explicit value instead of parent state.

Testing Details 🔍

• Ran swift test --filter UnitTests successfully.