Add button to select all text in a preformatted block

[JammyDodger]

Ceci est maintenant non seulement disponible dans le cœur, mais aussi activé par défaut.

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Just a little extra announcement to append to this… The setting has proved to be so useful and universally lauded that it is now not only default on but also tucked away as a hidden site setting: https://github.com/discourse/discourse/commit/81f3f56deb9d9f363e8b3bdcf81306fc34751d7d