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/Microsoft.CmdPal.UI.ViewModels/ProviderSettings.cs

@@ -10,6 +10,8 @@ public class ProviderSettings
 {
     public bool IsEnabled { get; set; } = true;
 
+    public Dictionary<string, bool> FallbackCommands { get; set; } = [];
+
     [JsonIgnore]
     public string ProviderDisplayName { get; set; } = string.Empty;
 
@@ -42,4 +44,14 @@ public class ProviderSettings
             throw new InvalidDataException("Did you add a built-in command and forget to set the Id? Make sure you do that!");
         }
     }
+
+    public bool IsFallbackEnabled(TopLevelViewModel command)
+    {
+        return FallbackCommands.TryGetValue(command.Id, out var enabled) ? enabled : true;
+    }
+
+    public void SetFallbackEnabled(TopLevelViewModel command, bool enabled)
+    {
+        FallbackCommands[command.Id] = enabled;
+    }
 }