Add support for secondary placeholders for singular and plural specification in label autofill

Originally from: https://github.com/WebDevStudios/custom-post-type-ui/issues/920

When translating auto-populated string values, we want to add support for returning either {{singular}} or {{plural}} instead of %s for placeholders. PHP won't throw a fit if no %s placeholder is returned, with the replacement value still provided. It would if the replacement value wasn't provided.

So in our javascript, we need to amend the section linked below, to first check for {{singular}} or {{plural}} first and if either of those exist, replace appropriately OTHERWISE just use what we have now with checking the if ( 'plural' === plurality ) { etc.

Make sure the description/helper text reflects the availability for these placeholders.

https://github.com/WebDevStudios/custom-post-type-ui/blob/1.13.4/src/js/cptui-scripts.js#L259-L274

Start in src/js/cptui-scripts.js around lines 259-274 and trace the label autofill replacement logic for singular and plural values. Update the related description/helper text to mention the {{singular}} and {{plural}} placeholders, then verify that translated labels use those placeholders when present and preserve the existing fallback behavior otherwise.