Move views_handler_field_custom_pre_render_move_text function to a class method on Drupal\views\Plugin\views\field\Custom

This is reliant on #1993312: Change pre_render and post_render callbacks to accept callables, but once in we can attach and render callbacks to plugins/handlers. This seems generally a nice idea to me. This function is only applicable to the Drupal\views\Plugin\views\field\Custom handler, but currently lives in views.module. Just doesn't make sense. Patch attached containing #1993312 also, if people want to test.

Steps to test:

- Add a 'Global: Custom text' field handler
- On the handler edit form 'Text' textarea should appear directly below the 'exclude from display' checkbox (screenshot attached)