Refactor Expander.ContentHeight and ExpanderAnimationBehavior

[stephenquan]

Description of Change

• Refactor/Rewrite Expander to expose Expander.ContentHeight
• Created ExpanderAnimationBehavior that animates the ContentHeight

Linked Issues

• Fixes https://github.com/CommunityToolkit/Maui/issues/2521

Linked Discussions

• https://github.com/CommunityToolkit/Maui/discussions/1628

PR Checklist

• [x] Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• [ ] Has tests (if omitted, state reason in description): TODO
• [x] Has samples (if omitted, state reason in description)
• [x] Rebased on top of main at time of PR
• [x] Changes adhere to coding standard
• [ ] Documentation created or updated: TODO

Additional information

This PR was created to solicit further suggestions from the team. This PR is a continuation of the discussion that began in this obsolete PR https://github.com/CommunityToolkit/Maui/pull/2522

[stephenquan]

@VladislavAntonyuk, I have recreated this PR based on your feedback in https://github.com/CommunityToolkit/Maui/pull/2522#issuecomment-2805516787