Rollup merge of #147396 - GuillaumeGomez:fluent-tidy-improvements, r=kobzol

Fluent tidy improvements

Follow-up of rust-lang/rust#147345 and of rust-lang/rust#147191.

It uses `fluent_syntax` to parse `fluent` files (but not for blessing, not even sure how the current one works).

I also added an `assert` to ensure we never go to previous situation where the `fluent` files were actually not checked at all.

cc ``@Kivooeo``
r? kobzol

Author: matthiaskrgr