Validate strings with placeholders in `make-mo`/`make-php`

[swissspidy]

This is a follow-up to #427

Ideally we add some validation logic to make-mo and make-php as well. We already do some validation in make-pot (see audit_strings() method, which we could extract into a reusable trait.

Maybe make-json too?

Prevents you from generating translation files with incorrect placeholders in them.