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/extensionsdk/Microsoft.CommandPalette.Extensions.Toolkit/Command.cs

@@ -18,7 +18,7 @@ public partial class Command : BaseObservable, ICommand
 
 = string.Empty;
 
-    public virtual string Id { get; protected set; } = string.Empty;
+    public virtual string Id { get; set; } = string.Empty;
 
     public virtual IconInfo Icon
     {