Issue #1785748 by yched, swentel, Stalski: Field formatters as plugins.

core/modules/field/field.api.php

core/modules/field/field.attach.inc

core/modules/field/field.default.inc

core/modules/field/field.info.inc

core/modules/field/field.module

core/modules/field/lib/Drupal/field/FieldInfo.php

core/modules/field/lib/Drupal/field/FieldInstance.php

.../lib/Drupal/field/Plugin/Type/Formatter/FormatterBase.php

...b/Drupal/field/Plugin/Type/Formatter/FormatterFactory.php

...Drupal/field/Plugin/Type/Formatter/FormatterInterface.php

...ugin/Type/Formatter/FormatterLegacyDiscoveryDecorator.php

...al/field/Plugin/Type/Formatter/FormatterPluginManager.php

...lib/Drupal/field/Plugin/Type/LegacyDiscoveryDecorator.php

...eld/Plugin/Type/Widget/WidgetLegacyDiscoveryDecorator.php

...b/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php

...b/Drupal/field/Plugin/field/formatter/LegacyFormatter.php

core/modules/field/lib/Drupal/field/Tests/FieldInfoTest.php

.../number/Plugin/field/formatter/DefaultNumberFormatter.php

.../number/Plugin/field/formatter/NumberDecimalFormatter.php

.../number/Plugin/field/formatter/NumberIntegerFormatter.php