patch 8.2.1485: Vim9: readdirex() expression doesn't accept bool

Problem:    Vim9: readdirex() expression doesn't accept bool.
Solution:   Accept both -1 and bool. (closes #6737)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 79fd80a..751cd6d 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1386,6 +1386,10 @@
   assert_equal(2, search('bar', 'W', 0, 0, {-> val == 1}))
 enddef
 
+def Test_readdirex()
+   eval expand('.')->readdirex({e -> e.name[0] !=# '.'})
+enddef
+
 def Fibonacci(n: number): number
   if n < 2
     return n