CmdPal: Plain text viewer and image viewer IContent (#43964)

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fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request

This PR introduces new types of IContent:  
- Plain text content – simple and straightforward, with options to
switch between UI and monospace fonts and toggle word wrap.
- It's super safe to display any random text content without having to
worry about escaping the pesky markdown.
- Image viewer content – a more polished way to display images:  
- When placed in the ContentPage, the component automatically resizes to
fit the viewport, ensuring the entire image is visible at once.
- Images can be opened in a built-in mini-viewer that lets you view,
pan, and zoom without leaving the Command Palette. (Doing this directly
on the page proved to be a UX and development headache.) Fully
keyboard-controllable, so there’s no need to take your hands off the
keys.

## Pictures? Pictures!

Plain text content:

<img width="960" height="604" alt="image"
src="https://github.com/user-attachments/assets/a4ec36f3-2f7f-4a2a-a646-53056c512023"
/>

Image viewer content:

<img width="939" height="605" alt="image"
src="https://github.com/user-attachments/assets/c87f5726-8cd0-4015-b2d9-f1457fa1ec96"
/>



https://github.com/user-attachments/assets/915cd9d2-e4e3-4baf-8df6-6a328a3592ba


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## PR Checklist

- [x] Closes: #41038
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **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

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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 -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml

@@ -92,7 +92,7 @@
                                 Height="16"
                                 Margin="0,3,8,0"
                                 SourceKey="{x:Bind Icon, Mode=OneWay}"
-                                SourceRequested="{x:Bind help:IconCacheProvider.SourceRequested20}" />
+                                SourceRequested="{x:Bind help:IconProvider.SourceRequested20}" />
                             <TextBlock Style="{StaticResource BodyTextBlockStyle}" Text="{x:Bind Name}" />
                         </StackPanel>
                     </Button>
@@ -301,7 +301,7 @@
                         ui:VisualExtensions.NormalizedCenterPoint="0.5,0.5"
                         Foreground="{ThemeResource TextFillColorSecondaryBrush}"
                         SourceKey="{x:Bind ViewModel.CurrentPage.Icon, Mode=OneWay}"
-                        SourceRequested="{x:Bind help:IconCacheProvider.SourceRequested20}"
+                        SourceRequested="{x:Bind help:IconProvider.SourceRequested20}"
                         Visibility="{x:Bind ViewModel.CurrentPage.IsRootPage, Mode=OneWay, Converter={StaticResource BoolToInvertedVisibilityConverter}}">
                         <animations:Implicit.ShowAnimations>
                             <animations:OpacityAnimation
@@ -434,7 +434,7 @@
                             HorizontalAlignment="Left"
                             AutomationProperties.AccessibilityView="Raw"
                             SourceKey="{x:Bind ViewModel.Details.HeroImage, Mode=OneWay}"
-                            SourceRequested="{x:Bind help:IconCacheProvider.SourceRequested64}"
+                            SourceRequested="{x:Bind help:IconProvider.SourceRequested64}"
                             Visibility="{x:Bind HasHeroImage, Mode=OneWay}" />
 
                         <TextBlock