Add animated content update support

[bryankeller]

Details

This adds animated setContent support to HorizonCalendar. Due to SwiftUI animation / UIKit-interop limitations, we don't currently attempt to animate from CalendarViewRepresentable. There's a hacky path forward if we mirror some private properties like this:

(lldb) po Mirror(reflecting: context.transaction.animation!).children.map { ($0, $1) }
▿ 1 element
  ▿ 0 : 2 elements
    ▿ .0 : Optional<String>
      - some : "base"
    ▿ .1 : BezierAnimation
      - duration : 0.35
      ▿ curve : BezierTimingCurve
        - ax : 0.52
        - bx : -0.78
        - cx : 1.26
        - ay : -2.0
        - by : 3.0
        - cy : 0.0

Related Issue

N/A

Motivation and Context

Improve animation support.

How Has This Been Tested

Example app.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[brynbodayle]

Nice!