How to Create a Custom Field Formatter in Drupal 8

In Drupal, field values are displayed to the end user via a field formatter. Formatters that come with fields are often pretty basic. If you want to change what a field displays, then the best way to do this is to write a custom formatter. A field formatter in Drupal 7 can be created in a few ways: first with code or using the Custom Formatters module. On the other hand, if you're using Display Suite, then you can create a "Display Suite Field" which is similar to a formatter. But now it's...