[WinUI, Shell] Enabling and Disabling ShellElements has no effect

### Description

Wire up Enabling and Disabling on Shell elements on WinUI.

We need to reconcile this behavior with this PR https://github.com/xamarin/Xamarin.Forms/pull/7847

Because currently the IsEnabled implementation seems to be incorrect. IsEnabled should do what IsVisible is promising on #7847 and the current IsEnabled implementation should map to attached properties (Shell.FlyoutItemDisabled, Shell.TabDisabled, etc..)

Currently IsEnabled doesn't even work for the top tabs it only works for bottom tabs and flyout items so the implementation as is is broken

Start by reading the linked PR #7847 and locating the Shell IsEnabled implementation. Compare its behavior for top tabs, bottom tabs, and flyout items with the attached-property behavior mentioned in the issue. Done means enabling and disabling Shell elements works consistently on WinUI without conflicting with the intended IsVisible behavior.

Written by the indexing model from the issue text.