Add a keyboard shortcut handler to the TabRowControl (#12260)

This makes the scenario mentioned in #8480 work. It's maybe not as holistic a solution as we'd like, but it definitely works.

Tested both with Narrator, and using the TabView scroll handles to get tab focus into the tab row manually

* [x] Closing the _active_ tab with <kbd>Ctrl+Shift+w</kbd> works (not the `TabViewItem` that has focus, but that's how Edgium works so that seems fine)
* [x] Opening a tab with <kbd>Ctrl+Shift+t</kbd> works
* [x] Opening the cmdpal with <kbd>Ctrl+Shift+p</kbd> works
* [x] Will take care of #8480 once we get the a11y team to validate
* [x] I work here


#### Notes:

None of

```xaml
PreviewKeyDown="_KeyDownHandler"
KeyDown="_KeyDownHandler"
KeyUp="_KeyDownHandler"
```

On the TerminalPage directly seem to fire when the focus is in the `TabViewItem` or the New Tab flyout. But they fire just fine when focus is in the `TermControl`. Interesting, because you'd think that the `TermControl` would have already handled the key...

src/cascadia/TerminalApp/TabBase.cpp

@@ -251,4 +251,5 @@ namespace winrt::TerminalApp::implementation
             }
         });
     }
+
 }

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -1169,6 +1169,12 @@ namespace winrt::TerminalApp::implementation
 
     // Method Description:
     // - Called when the users pressed keyBindings while CommandPalette is open.
+    // - As of GH#8480, this is also bound to the TabRowControl's KeyUp event.
+    //   That should only fire when focus is in the tab row, which is hard to
+    //   do. Notably, that's possible:
+    //   - When you have enough tabs to make the little scroll arrows appear,
+    //     click one, then hit tab
+    //   - When Narrator is in Scan mode (which is the a11y bug we're fixing here)
     // - This method is effectively an extract of TermControl::_KeyHandler and TermControl::_TryHandleKeyBinding.
     // Arguments:
     // - e: the KeyRoutedEventArgs containing info about the keystroke.

src/cascadia/TerminalApp/TerminalPage.xaml

@@ -21,7 +21,8 @@
         </Grid.RowDefinitions>
 
         <local:TabRowControl x:Name="TabRow"
-                             Grid.Row="0" />
+                             Grid.Row="0"
+                             KeyUp="_KeyDownHandler" />
 
         <StackPanel Grid.Row="1">
             <mux:InfoBar x:Name="KeyboardServiceWarningInfoBar"