seems like 'NormalYankDelete' functions need to yank one character more than the deletion range, so implement that, and make note to add tests for that module

fcore/normal-mode/normal-yank-delete.sml

@@ -6,7 +6,7 @@ structure NormalYankDelete =
 
        fun initMsgs (low, length, buffer) =
          let
-           val str = LineGap.substring (low, length, buffer)
+           val str = LineGap.substring (low, length + 1, buffer)
            val msg = YANK str
          in
            [DRAW msg]

todo.md

@@ -1,3 +1,4 @@
 # To-do list
 - Bind gamepad functions from GLFW and/or RGFW
 - Implement 'yj' motion and add tests for it
+- Add tests for NormalYankDelete functions, to make sure that they are yanking the expected string.