Add a "nop" command

The "nop" command, as the name implies, doesn't do anything.

The change isn't entirely frivolous ;-) Sending a "nop" command
interrupts any in-progress "prev-next" sequence without any other side
effects. Until a sequence is interrupted, all next/prev-window commands
navigate the windows in the LRU order captured at the time of the
first command in the sequence.

A "nop" command can be useful if bound to a particular key _release_
event in Sway. For instance, if <Mod1-release> is bound to "nop" while
<Mod1-Tab> and <Mod1-Shift-Tab> are bound to the "prev/next-window"
commands, then releasing <Mod1> will be enough to refresh the LRU
sequence for the next use of these next/prev bindings. This behavior
feels more intuitive (at least to me) than the non-nop variation.

One caveat though: there is currently an [outstanding bug in sway on
release bindings](https://github.com/swaywm/sway/issues/6456) which
prevents the described scenario from working exactly as described
because the Tab/S-Tab key presses actually cancel the outstanding
release action. There is however also a (non-merged) PR available that
addresses the issue.

swayr/src/cmds.rs

@@ -67,6 +67,9 @@ pub enum ConsiderWindows {
 
 #[derive(clap::Parser, Debug, Deserialize, Serialize)]
 pub enum SwayrCommand {
+    /// No-operation. Interrupts any in-progress prev/next sequence but has
+    /// no other effect
+    Nop,
     /// Switch to next urgent window (if any) or to last recently used window.
     SwitchToUrgentOrLRUWindow,
     /// Switch to the given app (given by app_id or window class) if that's not
@@ -264,6 +267,7 @@ pub fn exec_swayr_cmd(args: ExecSwayrCmdArgs) {
     }
 
     match args.cmd {
+        SwayrCommand::Nop => {}
         SwayrCommand::SwitchToUrgentOrLRUWindow => {
             switch_to_urgent_or_lru_window(&*props.read().unwrap())
         }