macOS: weak self for event monitor to avoid retain cycle for controllers (#4128)

Fixes #3219

We were holding a reference cycle to the base terminal controller. This
was preventing the window from ever being fully deallocated.

macos/Sources/Features/Terminal/BaseTerminalController.swift

@@ -111,8 +111,8 @@ class BaseTerminalController: NSWindowController,
         // Listen for local events that we need to know of outside of
         // single surface handlers.
         self.eventMonitor = NSEvent.addLocalMonitorForEvents(
-            matching: [.flagsChanged],
-            handler: localEventHandler)
+            matching: [.flagsChanged]
+        ) { [weak self] event in self?.localEventHandler(event) }
     }
 
     deinit {