Implement View.border modifier

[stackotter]

Here's the docs for corresponding SwiftUI modifier: https://developer.apple.com/documentation/swiftui/view/border(_:width:)

It takes a some ShapeStyle as its fill, but we don't support that yet, so our modifier should just take a Color for now.

This modifier is useful in general, but should be especially useful for helping people debug their layouts.

We'll need to make sure that it works nicely with corner radius, but that could be difficult given that we don't have a generic clip shape system yet.