Remove BindableObject Constraint from .Assign() and .Invoke()

[brminnick]

Description of Change

This PR removes the BindableObject constraint (where T : BindableObject) from .Assign() and .Invoke() extension methods.

Before

public static TBindable Assign<TBindable, TVariable>(this TBindable bindable, out TVariable variable) 
  where TBindable : BindableObject, TVariable;

public static TBindable Invoke<TBindable>(this TBindable bindable, Action<TBindable> action) 
  where TBindable : BindableObject;

After

public static TAssignable Assign<TAssignable, TVariable>(this TAssignable assignable, out TVariable variable) 
  where TAssignable : TVariable;

public static TAssignable Invoke<TAssignable>(this TAssignable assignable, Action<TAssignable> action);
  // no constraint

Linked Issues

• Fixes [Proposal] Remove BindableObject Constraint from .Assign() and .Invoke() #130

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 (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: [Maui.Markup] Add ObjectExtension Docs MicrosoftDocs/CommunityToolkit#161