feat: Removing Uno.Extensions.Toolkit as uno.toolkit no longer referenced

[nickrandolph]

GitHub Issue (If applicable): #1627

PR Type

What kind of change does this PR introduce?

• Refactoring (no functional changes, no api changes)

What is the current behavior?

Uno.Extensions.Toolkit/Toolkit.UI/Toolkit.WinUI is confusing since the packages no longer require uno.toolkit

What is the new behavior?

Code is moved to Uno.Extensions.Core/Core.UI/Core.WinUI
HostBuilderExtensions moved to Uno.Extensions.Hosting.UI/WinUI

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• [N/A] Docs have been added/updated which fit documentation template. (for bug fixes / features)
• [N/A] Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• [N/A] Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Updated the Release Notes
• Associated with an issue (GitHub or internal)

Other information

Internal Issue (If applicable):

[nickrandolph]

@lukeblevins can we add to v5 migration documentation:

• Remove Uno.Extensions.Toolkit and Uno.Extensions.Toolkit.UI/WinUI package references (no longer required)
• Remove Uno.Extensions package reference (no longer required)