Add doom/set-frame-opacity command

I use this to play videos behind Emacs while I work.

core/autoload/ui.el

@@ -150,6 +150,17 @@ set."
     (set-frame-font doom-font t))
   (doom|init-fonts))
 
+;;;###autoload
+(defun doom/set-frame-opacity (opacity)
+  "Interactively change the current frame's opacity.
+
+OPACITY is an integer between 0 to 100, inclusive."
+  (interactive
+   (list (read-number "Opacity (0-100): "
+                      (or (frame-parameter nil 'alpha)
+                          100))))
+  (set-frame-parameter nil 'alpha opacity))
+
 
 ;;
 ;; Modes