Add a [quantity_step] placeholder to the Step validation error message

[jankohoener]

Currently in the Step validation error message, the only placeholders are [should_min] and [should_next]. However, I find the message not very helpful, because nowhere does it get clear for the customer that a product has to be bought in steps of [quantity_step]. Rather than using the default message, I'd rather use a message like "This product can only be bought in steps of [quantity_step]. The nearest valid values are [should_min] and [should_next]."
But that placeholder isn't available yet and I'd be glad to have it.

[Fazle Bari]

Thanks for your suggestion, we will add quantity_step placeholders.

[Saiful Islam]

Dear @jankohoener, Thanks for your suggestion, I will add this feature right now. I am just working right now.

[Saiful Islam]

Finally done and added more Shortcode for Message for all Message.
Such for Step Variation Message: Available shortcode [should_min],[should_next],[product_name],[variation_name],[quantity],[min_quantity],[step_quantity]