Manually select a new tab when we're in fullscreen mode (microsoft#5809)

If we're fullscreen, the TabView isn't `Visible`. If it's not `Visible`,
it's _not_ going to raise a `SelectionChanged` event, which is what we
usually use to focus another tab. Instead, we'll have to do it manually
here.

So, what we're going to try to do is move the focus to the tab to the
left, within the bounds of how many tabs we have.

EX: we have 4 tabs: [A, B, C, D]. If we close:
* A (`tabIndex=0`): We'll want to focus tab B (now in index 0)
* B (`tabIndex=1`): We'll want to focus tab A (now in index 0)
* C (`tabIndex=2`): We'll want to focus tab B (now in index 1)
* D (`tabIndex=3`): We'll want to focus tab C (now in index 2)

`_UpdatedSelectedTab` will do the work of setting up the new tab as the
focused one, and unfocusing all the others.

Also, we need to _manually_ set the SelectedItem of the tabView here. If
we don't, then the TabView will technically not have a selected item at
all, which can make things like ClosePane not work correctly.

## PR Checklist
* [x] Closes microsoft#5799
* [x] I work here
* [ ] Tests added/passed
* [n/a] Requires documentation to be updated

## Validation Steps Performed

Played with it a bunch

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -854,6 +854,37 @@ namespace winrt::TerminalApp::implementation
         {
             _lastTabClosedHandlers(*this, nullptr);
         }
+        else if (_isFullscreen)
+        {
+            // GH#5799 - If we're fullscreen, the TabView isn't visible. If it's
+            // not Visible, it's _not_ going to raise a SelectionChanged event,
+            // which is what we usually use to focus another tab. Instead, we'll
+            // have to do it manually here.
+            //
+            // We can't use
+            //   auto selectedIndex = _tabView.SelectedIndex();
+            // Because this will always return -1 in this scenario unfortunately.
+            //
+            // So, what we're going to try to do is move the focus to the tab
+            // to the left, within the bounds of how many tabs we have.
+            //
+            // EX: we have 4 tabs: [A, B, C, D]. If we close:
+            // * A (tabIndex=0): We'll want to focus tab B (now in index 0)
+            // * B (tabIndex=1): We'll want to focus tab A (now in index 0)
+            // * C (tabIndex=2): We'll want to focus tab B (now in index 1)
+            // * D (tabIndex=3): We'll want to focus tab C (now in index 2)
+            const auto newSelectedIndex = std::clamp<int32_t>(tabIndex - 1, 0, _tabs.Size());
+            // _UpdatedSelectedTab will do the work of setting up the new tab as
+            // the focused one, and unfocusing all the others.
+            _UpdatedSelectedTab(newSelectedIndex);
+
+            // Also, we need to _manually_ set the SelectedItem of the tabView
+            // here. If we don't, then the TabView will technically not have a
+            // selected item at all, which can make things like ClosePane not
+            // work correctly.
+            auto newSelectedTab{ _GetStrongTabImpl(newSelectedIndex) };
+            _tabView.SelectedItem(newSelectedTab->GetTabViewItem());
+        }
     }
 
     // Method Description: