Allow the use of regex `\b` for word boundary (and `\B` for the opposite) even when using PostgreSQL.
Follow up of:
* https://github.com/FreshRSS/FreshRSS/pull/6706
For instance, `intitle:/\bnew\B/` will find *newest* but not *new* nor *renewal*.
Useful in particular to minimise the differences between PHP and database in:
* https://github.com/FreshRSS/FreshRSS/pull/7959