Create IPlaceholder Extensions

[brminnick]

Description of Change

This PR implements the following fluent extension methods for IPlaceholder:

public static TBindable PlaceholderColor<TBindable>(this TBindable bindable, Color? textColor) where TBindable : BindableObject, IPlaceholder;
public static TBindable Placeholder<TBindable>(this TBindable bindable, string? text) where TBindable : BindableObject, IPlaceholder;
public static TBindable Placeholder<TBindable>(this TBindable bindable, string? text, Color? textColor) where TBindable : BindableObject, IPlaceholder;

Linked Issues

• Fixes [Proposal] Add IPlaceholder Extension Methods #43

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: Add IPlaceholder Extensions Docs for Maui.Markup MicrosoftDocs/CommunityToolkit#48

Additional information

This PR also adds the .Placeholder() to the sample app and includes Unit Tests for the new IPlaceholder extension methods.

This PR also updates the README, adding the workflow image to match the README in CommunityToolkit/Maui.

[pictos]

LGTM!