patch 8.2.1576: Vim9: index() does not take "true" as argument

Problem:    Vim9: index() does not take "true" as argument.
Solution:   Use tv_get_bool_chk(). (closes #6823)

Author: brammool

src/evalfunc.c

@@ -4944,7 +4944,7 @@ f_index(typval_T *argvars, typval_T *rettv)
 	    item = list_find(l, (long)tv_get_number_chk(&argvars[2], &error));
 	    idx = l->lv_u.mat.lv_idx;
 	    if (argvars[3].v_type != VAR_UNKNOWN)
-		ic = (int)tv_get_number_chk(&argvars[3], &error);
+		ic = (int)tv_get_bool_chk(&argvars[3], &error);
 	    if (error)
 		item = NULL;
 	}

src/testdir/test_vim9_func.vim

@@ -1497,6 +1497,10 @@ def Test_count()
   assert_equal(0, count('ABC ABC ABC', 'b', false))
 enddef
 
+def Test_index()
+  assert_equal(3, index(['a', 'b', 'a', 'B'], 'b', 2, true))
+enddef
+
 def Test_expand()
   split SomeFile
   assert_equal(['SomeFile'], expand('%', true, true))

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1576,
 /**/
     1575,
 /**/