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)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 340ecf0..4f0bb5c 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -4944,7 +4944,7 @@
 	    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;
 	}