Improve FieldItemBase class

Problem/Motivation

`FieldItemBase::getEntity()` and `getLangcode()` both use the return of `getParent()` without checking whether it is Null.

`FieldItemBase` functions use `$this->definitions` while there is a perfectly good `getDataDefinition()` call which returns it and IMO should be used.

FieldItemBase could benefit from having 'replica' copies of `getPropertyDefinition($n)` and `getPropertyDefinitions()` calling through `getDataDefinition()`.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet