patch 8.1.1428: popup_atcursor() not implemented yet

Problem:    Popup_atcursor() not implemented yet.
Solution:   Implement it. (Yasuhiro Matsumoto, closes #4456)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index d1e89eb..a6703d1 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -809,6 +809,7 @@
     {"perleval",	1, 1, f_perleval},
 #endif
 #ifdef FEAT_TEXT_PROP
+    {"popup_atcursor",	2, 2, f_popup_atcursor},
     {"popup_close",	1, 1, f_popup_close},
     {"popup_create",	2, 2, f_popup_create},
     {"popup_getoptions", 1, 1, f_popup_getoptions},