Add button to select all text in a preformatted block

[JammyDodger]

Dies ist jetzt nicht nur in Core verfügbar, sondern auch standardmäßig aktiviert.

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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