Replace placeholders with example numbers when pretranslating plurals

[mathjazz]

Later on, we should start replacing the formatted { $count } placeholders in what we're sending with some actually appropriate numbers for each category, and then re-integrating the placeholders in what comes back.

Originally posted by @eemeli in https://github.com/mozilla/pontoon/pull/2802#pullrequestreview-1347961964