patch 8.2.1141: Vim9: return type of filter() is any

Problem:    Vim9: return type of filter() is any.
Solution:   Use type of the argument.
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 525d46d..8b83d77 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -918,6 +918,15 @@
   assert_equal(6, res)
 enddef
 
+def Test_filter_return_type()
+  let l = filter([1, 2, 3], {-> 1})
+  let res = 0
+  for n in l
+    res += n
+  endfor
+  assert_equal(6, res)
+enddef
+
 def Line_continuation_in_def(dir: string = ''): string
     let path: string = empty(dir)
             \ ? 'empty'