Support non-JSON config properties (such as callbacks and regexes) through config/ckeditor.js

[timraasveld]

This PR allows creating a config/ckeditor.js (or .ts) in a Strapi project to as a replacement for the editor: section in config/plugins.ts.

In config/plugins.ts, properties which are non serializable, such as regexes and callbacks, would get truncated when fetching the config. This makes it impossible to configure, for example, mediaEmbed providers.

This PR tackles the problem by having a config/ckeditor.js inserted in a <script> tag, and assigning that to the CKEditor config.

Example use case to make mediaEmbed use youtube-nocookie.com when a YT video is inserted:

// config/ckeditor.js
globalThis.ckEditorConfig = {
    toolbar: {
        items: [
          ...
        ]
    },
    mediaEmbed: {
        previewsInData: true,

        providers: [
            {
                name: 'youtube',
                url: [
                    /^(?:m\.)?youtube\.com\/watch\?v=([\w-]+)(?:&t=(\d+))?/,
                    /^(?:m\.)?youtube\.com\/v\/([\w-]+)(?:\?t=(\d+))?/,
                    /^youtube\.com\/embed\/([\w-]+)(?:\?start=(\d+))?/,
                    /^youtu\.be\/([\w-]+)(?:\?t=(\d+))?/
                ],
                html: match => {
                    const id = match[1];
                    
                    return (`<iframe
                              src="https://www.youtube-nocookie.com/embed/${id}"
                              frameborder="0"
                              allow="autoplay; encrypted-media" 
                              allowfullscreen />
                        `);
                }
            },
        ]
    }
}