bug#56767: New commands to split frame

[Juri Linkov]

Since we already have the commands to split the current window,
shouldn't we also have the commands to split the current frame:

```
diff --git a/lisp/window.el b/lisp/window.el
index 4d88ffa903..86dec25cdf 100644
--- a/lisp/window.el
+++ b/lisp/window.el
@@ -5754,6 +5754,17 @@ split-window-right
     new-window))
 
 (defalias 'split-window-horizontally 'split-window-right)
+
+(defun split-frame-vertically (&optional size)
+  "Like `split-window-vertically' but splits frame instead of window."
+  (interactive "P")
+  (split-window (frame-root-window) size))
+
+(defun split-frame-horizontally (&optional size)
+  "Like `split-window-horizontally' but splits frame instead of window."
+  (interactive "P")
+  (split-window (frame-root-window) size t))
+
 
 ;;; Balancing windows.
 
```