macos: swiftlint 'no_fallthrough_only' rule

This rule is generally trying to be helpful, but it doesn't like a few
places in our code base where we're intentionally listing out all of the
well-known cases. Given that, just disable it.

https://realm.github.io/SwiftLint/no_fallthrough_only.html

macos/Sources/App/macOS/AppDelegate.swift

@@ -380,13 +380,7 @@ class AppDelegate: NSObject,
 
             if let why = event.attributeDescriptor(forKeyword: keyword) {
                 switch why.typeCodeValue {
-                case kAEShutDown:
-                    fallthrough
-
-                case kAERestart:
-                    fallthrough
-
-                case kAEReallyLogOut:
+                case kAEShutDown, kAERestart, kAEReallyLogOut:
                     return .terminateNow
 
                 default: