patch 8.2.1603: Vim9: cannot use "true" with getchar()

Problem:    Vim9: cannot use "true" with getchar().
Solution:   use tv_get_bool_chk(). (closes #6874)

src/testdir/test_vim9_func.vim

@@ -1505,6 +1505,10 @@ def Test_getbufinfo()
   bwipe Xtestfile1 Xtestfile2
 enddef
 
+def Test_getchar()
+  assert_equal(0, getchar(true))
+enddef
+
 def Fibonacci(n: number): number
   if n < 2
     return n