patch 8.2.1610: Vim9: cannot pass "true" to list2str() and str2list()

Problem:    Vim9: cannot pass "true" to list2str() and str2list().
Solution:   Use tv_get_bool_chk(). (closes #6877)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index a80767a..84fc7c9 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1522,6 +1522,13 @@
   assert_equal(1, has('eval', true))
 enddef
 
+def Test_list2str_str2list_utf8()
+  let s = "\u3042\u3044"
+  let l = [0x3042, 0x3044]
+  assert_equal(l, str2list(s, true))
+  assert_equal(s, list2str(l, true))
+enddef
+
 def Fibonacci(n: number): number
   if n < 2
     return n