Add helper method to get enum instances of a multiple field

Problem/Motivation

It's already possible to get the enum instance of a single field item:

$entity->get('some_taxonomy_enum_field')->enum

but it's not yet possible to get all enum instances of a field item list.

Proposed resolution

Add a new helper method to the field item list class:

$entity->get('some_taxonomy_enum_field')->enums()