Standardizing built-in extension icon handling (#40606)

Just standardizing built-in extensions to use a `internal sealed class
Icons` for all their non-dynamic icons.

Looks like a LOT of changes, but it's icons all the way down.

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.Apps/Programs/UWPApplication.cs

@@ -106,7 +106,6 @@ public class UWPApplication : IProgram
                 new OpenPathCommand(Location)
                 {
                     Name = Resources.open_containing_folder,
-                    Icon = new("\ue838"),
                 })
             {
                 RequestedShortcut = KeyChordHelpers.FromModifiers(ctrl: true, shift: true, vkey: VirtualKey.E),