Drupal 10 readiness

Problem/Motivation

The automated Drupal 10 compatibility patch from the bot seemed too loose.
We need to rework some things here to get a 2.1.0 release for Drupal 10 (and still supporting Drupal 9) and think of the future (3.0.0 release #3102317: [META] Roadmap).

As core will switch to CKEditor5, and we heavily depend on entity_embed, you cannot update to Drupal 10 version before they're ready too.
We will further support CKEditor 4, as we do not have a direct dependency to it, beside the SVG icon (which is already supported in CKEditor 4)
There is not much missing, see: #3272732: Drupal 10 & CKEditor 5 readiness

Remaining tasks

• Verify that this still runs on Drupal 9.3 to have the file_url_generator (I also choose to use Drupal.JS once - which depends on Drupal 9.2). Also run on PHP 7.3
• Decide the minimal version numbers of the required modules (embed, entity_embed)
• Implement an upgrad path for the icon (PNG 2 SVG)

And hey - it's running

You can setup your Drupal 10 composer.json to use the two modules repos like this:

"repositories": [
        {
            "type": "composer",
            "url": "https://packages.drupal.org/8",
            "exclude": [
                "drupal/media_directories",
                "drupal/entity_embed"
            ]
        },
        {
            "type": "git",
            "url": "git@git.drupal.org:issue/media_directories-3319214.git"
        },
        {
            "type": "git",
            "url": "git@git.drupal.org:issue/entity_embed-3272732.git"
        }
    ],

and with Drupal 10 beta2 you'll need these patches:

        "patches": {
            "drupal/core": {
                "#3313473 CKE5 derivable": "https://www.drupal.org/files/issues/2022-10-31/3313473-43.patch"
            },
            "drupal/entity_embed": {
              "#3287235-28: D10": "https://www.drupal.org/files/issues/2022-10-25/3287235-28.patch"
            },
            "drupal/embed": {
                "#3309747 CKE5": "https://git.drupalcode.org/project/embed/-/merge_requests/2.diff"
            }
        },

and then the composer require commands look like this:

composer require drupal/entity_embed:dev-3272732-drupal-10-
composer require drupal/media_directories:dev-3319214-drupal-10