Standardize on Tab::Close for closing tabs (#19656)

This makes it possible to subscribe to `Close` events.

src/cascadia/TerminalApp/Tab.cpp

@@ -933,6 +933,13 @@ namespace winrt::TerminalApp::implementation
         return res;
     }
 
+    void Tab::Close()
+    {
+        ASSERT_UI_THREAD();
+
+        Closed.raise(nullptr, nullptr);
+    }
+
     // Method Description:
     // - Prepares this tab for being removed from the UI hierarchy by shutting down all active connections.
     void Tab::Shutdown()

src/cascadia/TerminalApp/Tab.h

@@ -61,6 +61,7 @@ namespace winrt::TerminalApp::implementation
         void UpdateSettings(const winrt::Microsoft::Terminal::Settings::Model::CascadiaSettings& settings);
         void UpdateTitle();
 
+        void Close();
         void Shutdown();
         void ClosePane();
 

src/cascadia/TerminalApp/Tab.idl

@@ -22,6 +22,7 @@ namespace TerminalApp
         UInt32 TabViewNumTabs;
 
         void Focus(Windows.UI.Xaml.FocusState focusState);
+        void Close();
         void Shutdown();
 
         void SetDispatch(ShortcutActionDispatch dispatch);

src/cascadia/TerminalApp/TabManagement.cpp

@@ -411,13 +411,11 @@ namespace winrt::TerminalApp::implementation
         auto actions = t->BuildStartupActions(BuildStartupKind::None);
         _AddPreviouslyClosedPaneOrTab(std::move(actions));
 
-        _RemoveTab(tab);
+        tab.Close();
     }
 
-    // Method Description:
-    // - Removes the tab (both TerminalControl and XAML)
-    // Arguments:
-    // - tab: the tab to remove
+    // Removes the tab (both TerminalControl and XAML).
+    // NOTE: Don't call this directly, but rather `tab.Close()`.
     void TerminalPage::_RemoveTab(const winrt::TerminalApp::Tab& tab)
     {
         uint32_t tabIndex{};