Baseline Craft 5 CKEditor Field JSON Config

ckeditor-field-config.json

{
  "htmlSupport": {
    "allow": [
      {
        "attributes": true,
        "classes": [
          "button",
          "button-primary"
        ],
        "name": "a"
      }
    ]
  },
  "alignment": {
    "options": [
      "left",
      "right",
      "center"
    ]
  },
  "list": {
    "properties": {
      "reversed": false,
      "startIndex": false,
      "styles": false
    }
  },
  "link": {
    "addTargetToExternalLinks": false,
    "decorators": {
      "openInNewTab": {
        "attributes": {
          "rel": "noopener noreferrer",
          "target": "_blank"
        },
        "label": "Open in a new tab",
        "mode": "manual"
      }
    }
  }
}

Here's a good article on configuration compared to redactor. Updated the example to include the links in a new tab option.