Add button to select all text in a preformatted block

[JammyDodger]

这现在不仅在核心中可用，而且默认启用。

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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