return axis.labels values

Currently axis.labels API doesn't return labels as it says it does in the docs

I added this code here -

https://github.com/c3js/c3/blob/2f3cddb818065f1d0c45f562745d4249e12980ff/src/api.axis.ts#L13

return [$$.axis.owner.config.axis_y_label, $$.axis.owner.config.axis_y2_label];