Add visibleMonthRange and visibleDayRange to proxy

[bryankeller]

Details

Adds properties for getting the visible month and day range on CalendarViewProxy.

Related Issue

N/A

Motivation and Context

Filling a gap in the SwiftUI API

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.