[DX][D9] Add helper functions to inject/move items into a particular position in associative arrays

• The "insert" part of this request is basically the respective issue as https://www.drupal.org/project/drupal/issues/66183

  One of the most difficult things about manipulating Drupal forms is the process of manipulating the arrays themselves. And one of the things that I commonly want to do is "stick this stuff in after that". But I have yet to find a PHP function to manipulate arrays in this way.

  So I wrote this function called array_insert(). Perhaps there's a cleaner way to handle this, but it seems like a function that should be available in Drupal core. It would certainly help with a lot of common hook_form_alter() tasks.

  ...

• The "move" is something new (no equivalent d.org issue that I could find). I would like to be able to move an existing key in the array from its current position to the start/end of the array. So two new functions, like backdrop_array_move_key_to_start($array, $key)/backdrop_array_move_key_to_end($array, $key)

  I usually achieve the "move to start" with this:

  $my_array = array_merge(array($key => $my_array[$key]), $my_array);
  

  ...and "move to end" with this:

  $move = $my_array[$key];
  unset($my_array[$key]);
  $my_array[] = $move;
  

There could be more efficient/performant/readable ways, but it would be a much better DX if we did not have to search the web or previous code in Backdrop to find how to best do it. Helper functions with obvious names would be ideal for all that.