Option to show selected terms first

Problem/Motivation

I'm using the widget in a D10 site where most of the contents are migrated from a D7 site. I just implemented a media migration where media entities have a reference to a "tags" vocabulary. In this vocabulary (where the users can create their own values) exist over 2600 terms. It is really difficult to find which terms an entity references with such a long list if you have to scroll.

Steps to reproduce

• Create an empty D10 site with taxonomy_widget_plus and devel_generate modules enabled.
• Create a vocabulary and generate a large number of terms (several hundreds) with devel_generate.
• Add a taxonomy reference field to an entity type (i. e. the node type "article"), enabling the vocabulary created before.
• Create a new entity and select a few terms randomly from the long list of values and save.
• Edit the same entity and try to quickly find out which terms are referenced.

Proposed resolution

Add an option to the widget settings to show all selected terms at the top of the list if enabled.
If you decide to enable this option by default, an update hook is needed to set this option to disabled for all existing usages of the widget to keep the current behavior.

Remaining tasks

User interface changes

API changes

Data model changes