CKEditor 5 should respect <textarea disabled> dynamically

Problem/Motivation

With #3362386: CKEditor 5 should respect <textarea disabled>, the CKEditor is set to read-only mode during creation if the textarea is disabled.

But the textarea can also be disabled or enabled dynamically, e.g. via Conditional Form Fields.

Steps to reproduce

• Configure the #states property to dynamically set the disabled property (or use Conditional Fields) of a CKEditor field.
• Notice that the read-only mode of the CKEditor is only set once at the start, but not dynamically.

Proposed resolution

Use a MutationObserver to observe the disabled attribute of the textarea and set the read-only mode dynamically.

Remaining tasks

• implement tests
• implement solution

User interface changes

N/A

Introduced terminology

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

The CKEditor 5 read-only mode is now set dynamically dependent on the disabled attribute of the textarea. Previously, it was only set once at initialization.