CmdPal: Add settings to control which fallbacks are enabled (#40505)

This adds settings to each provider to allow us to control if individual
fallback items are enabled or not, regardless of the provider being
enabled.

This is relevant to _all the threads where disabling fallback commands
came up_

This just adds another section to each provider's settings page, with a
list of the fallback commands.

This also has nothing to do with the "top-level apps search", which is
not really a fallback command - it's its own thing.

Ref #38288. Doesn't close that, because this only controls
enable/disable, not ranking.

From here, we should be able to add a dedicated page in the SUI that
shows all the fallbacks across all providers. That's where we'll enable
the ordering.

src/modules/cmdpal/ext/Microsoft.CmdPal.Ext.WebSearch/FallbackExecuteSearchItem.cs

@@ -17,7 +17,7 @@ internal sealed partial class FallbackExecuteSearchItem : FallbackCommandItem
     private static readonly CompositeFormat PluginOpen = System.Text.CompositeFormat.Parse(Properties.Resources.plugin_open);
 
     public FallbackExecuteSearchItem(SettingsManager settings)
-        : base(new SearchWebCommand(string.Empty, settings), Resources.command_item_title)
+        : base(new SearchWebCommand(string.Empty, settings) { Id = "com.microsoft.websearch.fallback" }, Resources.command_item_title)
     {
         _executeItem = (SearchWebCommand)this.Command!;
         Title = string.Empty;