lint for discarding a value / error union

[paperdave]

        path_map.route_types.append(allocator, .{
            ...
        });

If the linter can know @TypeOf(path_map.route_types.append) to be a function that returns a non-void value (in my case an error union), it should immediately that this usage (where the result is discarded) will always be a compile error.