Add button to select all text in a preformatted block

[JammyDodger]

هذا متاح الآن ليس فقط في النواة، ولكنه ممكّن افتراضيًا أيضًا.

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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