patch 8.1.1522: poup_notification() not implemented yet

Problem:    Popup_notification() not implemented yet.
Solution:   Implement it.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 134280f..0ef7609 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -819,6 +819,7 @@
     {"popup_getpos",	1, 1, f_popup_getpos},
     {"popup_hide",	1, 1, f_popup_hide},
     {"popup_move",	2, 2, f_popup_move},
+    {"popup_notification", 2, 2, f_popup_notification},
     {"popup_show",	1, 1, f_popup_show},
 #endif
 #ifdef FEAT_FLOAT