Implement Keyboard property in EditorHandlers

[jsuarezruiz]

Description of Change

Implement Keyboard property in EditorHandlers.

Platforms Affected

• Core
• iOS
• macOS
• Android

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

[rachelkang]

I'm wondering - can we add the Windows implementation to this PR as well?

Also, there are failing iOS tests - not yet sure if any are related, but thinking the Editor ones might be

[rachelkang]

See above comment!