AmpTextFormatter (amp_text) cannot be applied to string_long fields

Problem/Motivation

AMP text formatter does not appear as an option on text fields of type string_long.

Steps to reproduce

Proposed resolution

Append string_long to AmpTextFormatter.php as follows

/**
 * Plugin implementation of the 'amp_text' formatter.
 *
 * @FieldFormatter(
 *   id = "amp_text",
 *   label = @Translation("AMP Text"),
 *   description = @Translation("Display AMP text."),
 *   field_types = {
 *     "string",
 *     "text",
 *     "text_long",
 *     "text_with_summary",
 *     "string_long"
 *   },
 * )
 */

Remaining tasks

User interface changes

API changes

Data model changes