vim-patch:8.1.1418: win_execute() is not implemented yet

Problem:    Win_execute() is not implemented yet.
Solution:   Implement it.
868b7b6712

src/nvim/eval.lua

@@ -390,6 +390,7 @@ return {
     visualmode={args={0, 1}},
     wait={args={2,3}},
     wildmenumode={},
+    win_execute={args={2, 3}},
     win_findbuf={args=1},
     win_getid={args={0,2}},
     win_gettype={args={0,1}},