Add default value on anchor_tag field when no source field is assigned.

Problem/Motivation

When editing / adding a paragraph with an anchor_tag with no source field assigned, I want the anchor_tag field to have a default value such as "paragraph-{$paragraph->id()}".

Steps to reproduce

Proposed resolution

Set a default value for anchor_tag when no source is assigned.

Remaining tasks

User interface changes

API changes

Data model changes