Rewrote js/mage/adminhtml/input-counter.js without prototypejs

[fballiano]

This PR targets next

This was very hard to test, I couldn't find anywhere where this file would originally be used (it's included in the html but it doesn't seem to run anywhere).

So

• I've created a product, with a "text" custom option, setting a maxlength for the custom option value
• I've created an order, trying to add the product from above, when the "configure" popup is shown then I'd copy past the inner part of the code from this PR (without the first line window.addEventListener('DOMContentLoaded', function() {)

The code is working as expected, this is the countdown when the input is still valid

and this is when it becomes invalid