Remove InternalsVisibleTo for Toolkit

[jfversluis]

Description of Change

Removes the InternalsVisibleTo for the .NET MAUI Community Toolkit. We shouldn't need that, it was done for historic reasons, but to prevent further unintentional breaking changes like #28450 we should remove it.

This PR:

• Removes the InternalsVisibleTo attribute from all the .NET MAUI projects in regard to the Toolkit
• Makes some internals that the Toolkit needs public
• Add XML comments to the APIs that are now public

Issues Fixed

Fixes #28981

[rmarinho]

/rebase

[jfversluis]

Built the .NET MAUI Community Toolkit against this version and seems like these are the APIs that are currently used, but are internal to .NET MAUI:

• IWindowController
• IPropertyPropagationController
• IResourcesProvider
• IStyleSelectable
• IStyleElement
• MergedStyle

From what I can determine the above ones are used in Popup, but will be removed with the new Popups implementation that is in a PR right now.

For the rest:

• ITextElement: AvatarView, I think we are OK with making this public
• ILineHeightElement: AvatarView, I think we are OK with making this public
• ITextAlignmentElement: AvatarView, I think we are OK with making this public
• ICornerElement: AvatarView, I think we are OK with making this public
• IProvideParentValues: AppThemeResourceExtension, we don't want to make this one public so created a PR with an alternative implementation on the Toolkit side: Update AppThemeResourceExtension to not use IProvideParentValues CommunityToolkit/Maui#2639

[dotnet-policy-service]

🚨 API change(s) detected @davidbritch FYI