Add button to select all text in a preformatted block

[JammyDodger]

This is now not only available in core, but also enabled by default.

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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