patch 8.2.1133: Vim9: return type of add() is not specific enough

Problem:    Vim9: return type of add() is not specific enough.
Solution:   Return the type of the first argument. (closes #6395)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 2cdf186..c3eaef2 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -441,7 +441,7 @@
 {
     {"abs",		1, 1, FEARG_1,	  ret_any,	FLOAT_FUNC(f_abs)},
     {"acos",		1, 1, FEARG_1,	  ret_float,	FLOAT_FUNC(f_acos)},
-    {"add",		2, 2, FEARG_1,	  ret_any,	f_add},
+    {"add",		2, 2, FEARG_1,	  ret_first_arg, f_add},
     {"and",		2, 2, FEARG_1,	  ret_number,	f_and},
     {"append",		2, 2, FEARG_LAST, ret_number,	f_append},
     {"appendbufline",	3, 3, FEARG_LAST, ret_number,	f_appendbufline},