Enforce unicode string usage

[ixzzd]

It's hard for people to remember to consistently use {js| |js} instead of "" in their localization phrases.
It is possible to match only unicode strings with this matcher Pconst_string (str, _, Some "js")

[davesnx]

The ppx can also transform any string to generate the Pconst_string with js, so no need for people to remember and just generate the right string for unicode.