New sample: Selectable TextBlock (#1578)

<!--- Provide a general summary of your changes in the Title above -->

## Description
As title suggest.

## Motivation and Context
Many people are not aware of `TextBlock` is selectable and use `TextBox`
instead which may have higher overhead. So I made this sample.

## How Has This Been Tested?
Mnaual

## Screenshots (if appropriate):

![image](https://github.com/microsoft/WinUI-Gallery/assets/42881734/cf154abe-6700-4b1b-90c0-695096fda9b4)


## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

WinUIGallery/ControlPages/TextBlockPage.xaml

@@ -79,5 +79,27 @@
                 </x:String>
             </local:ControlExample.Xaml>
         </local:ControlExample>
+
+        <local:ControlExample x:Name="Example5" HeaderText="A selectable TextBlock">
+            <local:ControlExample.Example>
+                <TextBlock IsTextSelectionEnabled="{x:Bind SelectionEnabledSwitch.IsOn, Mode=OneWay}"
+                           Text="I am a selectable TextBlock with custom SelectionHighlightColor."
+                           SelectionHighlightColor="DarkOrange"/>
+            </local:ControlExample.Example>
+
+            <local:ControlExample.Options>
+                <ToggleSwitch Header="IsTextSelectionEnabled" x:Name="SelectionEnabledSwitch"/>
+            </local:ControlExample.Options>
+            <local:ControlExample.Xaml>
+                <x:String xml:space="preserve">
+&lt;TextBlock IsTextSelectionEnabled="$(IsTextSelectionEnabled)"
+           Text="I am a selectable TextBlock with custom SelectionHighlightColor."
+           SelectionHighlightColor="DarkOrange"&gt;
+                </x:String>
+            </local:ControlExample.Xaml>
+            <local:ControlExample.Substitutions>
+                <local:ControlExampleSubstitution Key="IsTextSelectionEnabled" Value="{x:Bind SelectionEnabledSwitch.IsOn, Mode=OneWay}"/>
+            </local:ControlExample.Substitutions>
+        </local:ControlExample>
     </StackPanel>
 </Page>