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.WindowsTerminal/Icons.cs

@@ -0,0 +1,14 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CommandPalette.Extensions.Toolkit;
+
+namespace Microsoft.CmdPal.Ext.WindowsTerminal;
+
+internal sealed class Icons
+{
+    internal static IconInfo TerminalIcon { get; } = IconHelpers.FromRelativePath("Assets\\WindowsTerminal.svg");
+
+    internal static IconInfo AdminIcon { get; } = new IconInfo("\xE7EF"); // Admin icon
+}