Add a Field Widget for the embed_code Field Type in Media Embed Code

Problem/Motivation

The media_embed_code module does not provide a Field Widget for the embed_code field type.

Steps to reproduce

1. Enable the media_embed_code module.

2. Add an embed_code field and check the available widgets in Manage Form Display.

Proposed resolution

Implement a dedicated Field Widget plugin that supports only the embed_code field type.

Remaining tasks

1. Implement the widget.

2. Add automated tests.

User interface changes

Provide a suitable widget for entering and editing embed-code values.

API changes

Add a new Field Widget plugin.

Data model changes

None