Implement do-screen-transition action

niri-ipc/src/lib.rs

@@ -84,6 +84,12 @@ pub enum Action {
         #[cfg_attr(feature = "clap", arg(last = true, required = true))]
         command: Vec<String>,
     },
+    /// Do a screen transition.
+    DoScreenTransition {
+        /// Delay in milliseconds for the screen to freeze before starting the transition.
+        #[cfg_attr(feature = "clap", arg(short, long))]
+        delay_ms: Option<u16>,
+    },
     /// Open the screenshot UI.
     Screenshot,
     /// Screenshot the focused screen.