dwindle: add swapsplit dispatcher (#4702)

This is distinct from `swapwindow` in that it allows swapping the entire
tree node with its neighbour.

Fixes: https://github.com/hyprwm/Hyprland/issues/4701

src/managers/KeybindManager.hpp

@@ -127,6 +127,7 @@ class CKeybindManager {
     static void     alterSplitRatio(std::string);
     static void     focusMonitor(std::string);
     static void     toggleSplit(std::string);
+    static void     swapSplit(std::string);
     static void     moveCursorToCorner(std::string);
     static void     moveCursor(std::string);
     static void     workspaceOpt(std::string);