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/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index e8d712f..cc938a5 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -602,6 +602,12 @@
     res += n
   endfor
   assert_equal(3, res)
+
+  res = 0
+  for n in add([1, 2], 3)
+    res += n
+  endfor
+  assert_equal(6, res)
 enddef
 
 def Test_func_type_part()