Port Editor Placeholder text and color properties

[brunck]

Description of Change

Implements #388

Additions made

• Adds Placeholder (via the ITextView interface) and PlaceholderColor to the IEditor interface
• Adds Placeholder and PlaceholderColor to property map for EditorHandler
• Adds Placeholder and PlaceholderColor mapping methods to EditorHandler for Android and iOS
• Adds extension methods to update Placeholder and PlaceholderColor on Android/iOS
• Adds MauiTextView class for iOS to add placeholder functionality to UITextView
• Adds x-plat device tests to test initialization and updating of Placeholder and PlaceholderColor properties

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

[mattleibow]

Very very nice! Thanks for this PR.

I just got one change, but other than that... Merges will happen!

[mattleibow]

Nice.

[mattleibow]

I notice the placeholder is a bit off center... However, this seems to be an exact copy from Forms so might be a bug there. Not sure if we need to fix that here. But if it is an easy fix then maybe?

Maybe @PureWeen knows?