Create a Microsoft.CmdPal.Core.ViewModels project (#40560)

_targets #40504_ 

Major refactoring for #40113

This moves a large swath of the codebase to a `.Core` project. "Core"
doesn't have any explicit dependencies on "extensions", settings or the
current `MainListPage`. It's just a filterable list of stuff. This
should let us make this component a bit more reusable.

This is half of a PR. As I did this, I noticed a particular bit of code
for TopLevelVViewModels and CommandPaletteHost that was _very rough_.
Solving it in this PR would make "move everything to a new project" much
harder to review. So I'm submitting two PRs simultaneously, so we can
see the changes separately, then merge together.

src/modules/cmdpal/Microsoft.CmdPal.UI/ExtViews/ContentPage.xaml

@@ -9,6 +9,7 @@
     xmlns:cmdpalUI="using:Microsoft.CmdPal.UI"
     xmlns:controls="using:CommunityToolkit.WinUI.Controls"
     xmlns:converters="using:CommunityToolkit.WinUI.Converters"
+    xmlns:coreViewModels="using:Microsoft.CmdPal.Core.ViewModels"
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
     xmlns:labToolkit="using:CommunityToolkit.Labs.WinUI.MarkdownTextBlock"
     xmlns:local="using:Microsoft.CmdPal.UI"