feat: Make `fluent_syntax::parser::core::Parser` pub

[danielrainer]

This allows creating a simple FTL formatter by parsing and then immediately serializing the parsing result again.

[danielrainer]

I just played around with this some more and it turns out that with Parser accessible, quite a lot becomes possible. For example, I just built a POC tool for renaming IDs and variables on top of fluent-syntax.