patch 8.2.1606: Vim9: cannot use "true" with has()

Problem:    Vim9: cannot use "true" with has().
Solution:   Use tv_get_bool(). (closes #6876)

src/testdir/test_vim9_func.vim

@@ -1516,6 +1516,10 @@ def Test_getcompletion()
   set wildignore&
 enddef
 
+def Test_has()
+  assert_equal(1, has('eval', true))
+enddef
+
 def Fibonacci(n: number): number
   if n < 2
     return n