Provide a way for the “Add another item” button text on unlimited fields to be more easily configurable

Problem/Motivation

Core supplies the button text "Add another item" for add more buttons. It would be valuable to be able to customize this text. Our team had ideas to change it globally to something like "Add another" which is more concise, or to be able to pass in an entity name, for example a card entity could have the button text read "Add another card".

There is a way forward in the Simple Add More contrib module issue, Make interface text configurable, and I think it would be useful at the core level.

Steps to reproduce

* Create a content type with a field that has a fixed cardinality
* Observe that the button to add additional items to this field in the authoring form uses the text "Add another item"

Proposed resolution

Create a core-supported way to globally modify button text and help text used in the "Add another" for fixed-cardinality fields.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet