vim-patch:8.1.2020: it is not easy to change the window layout

Problem:    It is not easy to change the window layout.
Solution:   Add win_splitmove(). (Andy Massimino, closes vim/vim#4561)
d20dcb3d01

src/nvim/eval.lua

@@ -392,6 +392,7 @@ return {
     win_id2tabwin={args=1},
     win_id2win={args=1},
     win_screenpos={args=1},
+    win_splitmove={args={2, 3}},
     winbufnr={args=1},
     wincol={},
     windowsversion={},