feat(wm): add command to create new workspace

This commit adds a new command, 'komorebic.exe new-workspace', which
will append a new, empty workspace, to the list of workspaces on the
currently focused monitor, and then switch focus to it.

Also took the opportunity to clean up some unnecessary unwraps in
komorebic/src/main.rs.

resolve #4

komorebi/src/monitor.rs

@@ -128,6 +128,10 @@ impl Monitor {
         Ok(())
     }
 
+    pub fn new_workspace_idx(&self) -> usize {
+        self.workspaces().len()
+    }
+
     pub fn update_focused_workspace(&mut self) -> Result<()> {
         let work_area = *self.work_area_size();