Support "shift" and "pop" functionality in "extract" plugin

Oops. Changing back to 8.5.x.

Thanks for the feedback! Two things to note here:

• Unlike the extract plugin, the callback plugin doesn't "handle_multiple" values. So the solution in #8 is going to work on individual values separately and not treat them together as a whole (which is my use case)
• If the multiple values plugin (https://www.drupal.org/node/2901617) gets into migrate_plus, then #8 would make sense.

Please feel free to re-open if that makes sense.

I just tried to do the thing suggested in #8 and I remembered why I had to create a separate plugin called ArrayShift in my project in the first place. When you try to call array_pop or array_shift using the "callback" plugin, you get this message:

Parameter 1 to array_shift() expected to be a reference, value given Callback.php:54
Parameter 1 to array_pop() expected to be a reference, value given Callback.php:54

Hence, I'm reopening this issue. I think this is quite a common use case because I've stumbled upon such a problem 2 times in less than 3 months. I will see if I can write some tests for this this weekend.

Created 2 plugins - array_pop and array_shift. Do I need to include tests as well?

Done. Added tests for both the shift and pop plugins.

BTW, thinking aloud, I just thought of something different. Is there a reason why the callable plugin doesn't call the callable with arguments by reference? What if we had something like this in the callable plugin?

public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    if (is_callable($this->configuration['callable'])) {
      $callable = is_array($this->configuration['callable']) ? implode('::', $this->configuration['callable']) : $this->configuration['callable'];
      // Call by reference.
      if (isset($this->configuration['reference'])) {
        $callable($value);
      }
      // Call by value.
      else {
        $value = $callable($value);
      }
    }
    return $value;
  }

That way, people will be able to call a plethora of callbacks without having to write new plugins. Example: array_pop, array_shift, sort and all other functions which expect arguments by reference instead of value. Any thoughts? Also, I think the callback process plugin should throw an exception when the callback doesn't exist or is not callable instead of silently doing nothing. Let me know if this sounds good and I'll create a patch, with tests, etc to get the wheel rolling. I'm feeling highly motivated this week (:

I switched to a data provider except for the error test case where an exception in expected. I also fixed the documentation (which was caused by copy and paste). Examples are also improved now.

I am attaching a diff of the changes I made after your previous feedback to ensure you have something. I will figure out what an interdiff is and post it ASAP.

Update: I think what I posted with this comment is actually an interdiff. I had never heard the word before.

I am submitting a diff again for now. I saw the interdiff docs - thanks for that. I think I'll need to find some time to try that out sometime soon.

I have removed the comments and used array keys to define test context. However, I have used spaces instead of underscores in keys cause I think it's mostly for the UI only (which looks better without underscores). Let me know if it's better to use underscores and I'll change. Thanks again.

Update: I think the simpletest UI might have some kind of a problem. It shows this text (note the wrongly positioned parentheses):
Drupal\Tests\migrate_plus\Unit\process\ArrayShiftTest->testArrayShift with data set "indexed array"()

Ok. So Joe managed to submit his patch before I could upload mine. Great! Progress = Good.

Some coding standard fixes.