use a widget to select plugins instead of ID strings in a textfield

Problem/Motivation

At admin/config/content/insert, you have to enter plugin IDs in a textfield.

It would be much better DX to show two checkbox lists of plugin widget labels.

The Plugin module has an issue to provide a plugin form element: #3197304: Provide form element and reuse that in field widget

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes