patch 8.2.1142: Vim9: return type of insert() is any

Problem:    Vim9: return type of insert() is any.
Solution:   Use type of the first argument.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index b015f6d..65c34cd 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -667,7 +667,7 @@
     {"inputrestore",	0, 0, 0,	  ret_number,	f_inputrestore},
     {"inputsave",	0, 0, 0,	  ret_number,	f_inputsave},
     {"inputsecret",	1, 2, FEARG_1,	  ret_string,	f_inputsecret},
-    {"insert",		2, 3, FEARG_1,	  ret_any,	f_insert},
+    {"insert",		2, 3, FEARG_1,	  ret_first_arg, f_insert},
     {"interrupt",	0, 0, 0,	  ret_void,	f_interrupt},
     {"invert",		1, 1, FEARG_1,	  ret_number,	f_invert},
     {"isdirectory",	1, 1, FEARG_1,	  ret_number,	f_isdirectory},