Update BindablePropertySourceGenerator to use partial properties

[pictos]

Description of Change

Linked Issues

• Fixes [Housekeeping] Update BindableProperty SG to use partial Properties #2834

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: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

[Copilot]

Pull Request Overview

This PR updates the BindablePropertySourceGenerator to utilize C#'s partial properties feature, transitioning from class-level to property-level attribute application. The change modernizes the code generation pattern by allowing developers to declare bindable properties directly on partial properties rather than using class-level attributes.

Key changes:

• Modified the BindablePropertyAttribute to target properties instead of classes
• Updated the source generator to process property-level attributes and generate corresponding BindableProperty fields
• Refactored the Expander class to demonstrate the new partial property syntax

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
Expander.shared.cs	Converted class-level BindableProperty attributes to partial property declarations with property-level attributes
AttributeExtensions.cs	Enhanced constructor argument handling to support optional parameters with placeholders
BindablePropertyAttributeSourceGenerator.cs	Completely refactored to process property-level attributes and generate code for partial properties

[TheCodeTraveler]

This is awesome!! Thanks Pedro!