Add support for Font to Button and Entry

[mattleibow]

Description of Change

Implements the Font property (family, size, attributes) on Button and Entry. After the IFont was merged into IText in #416, we need to implement the other handlers that implement IText.

Additions made

• Adds Font property map to ButtonHandler and EntryHandler
• Adds Font mapping methods to ButtonHandler and EntryHandler for Android and iOS
• Adds extension methods to apply Font to ButtonHandler and EntryHandler on Android and iOS
• Adds DeviceTests for initial Font values to IButton and IEntry on iOS and 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

[rmarinho]

I see this on the build logs, why don0't the other builds fail?

/Users/runner/work/1/s/src/Core/src/Platform/iOS/ButtonExtensions.cs(37,17): error CS1061: 'UIButton' does not contain a definition for 'Font' and no accessible extension method 'Font' accepting a first argument of type 'UIButton' could be found (are you missing a using directive or an assembly reference?) [/Users/runner/work/1/s/src/Core/src/Core-net6.csproj]

[hartez]

I see this on the build logs, why don0't the other builds fail?

/Users/runner/work/1/s/src/Core/src/Platform/iOS/ButtonExtensions.cs(37,17): error CS1061: 'UIButton' does not contain a definition for 'Font' and no accessible extension method 'Font' accepting a first argument of type 'UIButton' could be found (are you missing a using directive or an assembly reference?) [/Users/runner/work/1/s/src/Core/src/Core-net6.csproj]

I'm guessing it's because Font is deprecated; we should be using UIButton.TitleLabel.Font. I'm fixing it now.