CmdPal: Extension list search (#41453)

## Summary of the Pull Request

Adds a rough implementation that appears to work, but it hasn’t been
tested. Don’t rely on it yet—I’m too tired to test properly or write a
polished commit message.

Pictures? Pictures!

<img width="2720" height="1624" alt="image"
src="https://github.com/user-attachments/assets/4f9c3360-8d08-4a34-888f-c6890afd0159"
/>

<img width="2647" height="1558" alt="image"
src="https://github.com/user-attachments/assets/3d6c23bc-f26d-4e31-ae09-d91efd304048"
/>

<img width="2683" height="1536" alt="image"
src="https://github.com/user-attachments/assets/6017b2d4-ccd9-4555-b81f-e2b1f78c48e5"
/>

<img width="577" height="334" alt="image"
src="https://github.com/user-attachments/assets/987fea99-05b2-4d31-b90c-327122daa241"
/>


https://github.com/user-attachments/assets/65a8f8ce-ae09-4272-881d-0b6f56cecfae

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] Closes: #41451
- [x] Closes: #42031
- [x] Closes: #41800
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
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---------

Co-authored-by: Niels Laute <niels.laute@live.nl>
Co-authored-by: Michael Jolley <mike@baldbeardedbuilder.com>

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/SettingsViewModel.cs

@@ -140,6 +140,8 @@ public partial class SettingsViewModel : INotifyPropertyChanged
 
     public ObservableCollection<ProviderSettingsViewModel> CommandProviders { get; } = [];
 
+    public SettingsExtensionsViewModel Extensions { get; }
+
     public SettingsViewModel(SettingsModel settings, IServiceProvider serviceProvider, TaskScheduler scheduler)
     {
         _settings = settings;
@@ -155,6 +157,8 @@ public partial class SettingsViewModel : INotifyPropertyChanged
             var settingsModel = new ProviderSettingsViewModel(item, providerSettings, _serviceProvider);
             CommandProviders.Add(settingsModel);
         }
+
+        Extensions = new SettingsExtensionsViewModel(CommandProviders, scheduler);
     }
 
     private IEnumerable<CommandProviderWrapper> GetCommandProviders()