CmdPal: fix missing builtin icons (#41298)

It would seem that the way we absorb the icons for built-in extension
into our package relies on the _extension_ package including WASDK. I
don't fully understand why.

This PR adds a common `.props` file we can use for all extensions, to
make sure they include it.

regressed in #41261
Closes #41279

src/modules/cmdpal/ext/Common.ExtDependencies.props

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project>
+
+<!-- 
+  Common external dependencies for all CmdPal extensions:
+  - Microsoft.WindowsAppSDK
+  - Microsoft.Web.WebView2
+  - Microsoft.CommandPalette.Extensions.Toolkit (via project reference)
+
+  We need the WASDK reference because without it, our image assets won't get
+  placed into our final package correctly. 
+
+-->
+    
+  <ItemGroup>
+    <PackageReference Include="Microsoft.WindowsAppSDK" />
+    <PackageReference Include="Microsoft.Web.WebView2"  />
+  </ItemGroup>
+  <ItemGroup>
+    <ProjectReference Include="..\..\extensionsdk\Microsoft.CommandPalette.Extensions.Toolkit\Microsoft.CommandPalette.Extensions.Toolkit.csproj" />
+  </ItemGroup>
+</Project>