Dll integration

[jodator]

Suggested merge commit message (convention)

Other: Enabled creating builds that can be extended (with more plugins) without the need to being recompiled. This required splitting the project into the, so-called, DLL part and consumers of this DLL. Under the hood, the mechanism is based on webpack DLLs. This is the first part of the required changes and it contains the necessary breaking changes (see the "MAJOR BREAKING CHANGES" section above). For more information see the "DLL builds" guide. Closes #8395.

Other: Introduced Image.isImageWidget() utility method.

Feature (core): Plugin collection will allow to require plugin by name when it is provided in config.plugins or if it was already loaded. Closes #2907.

MAJOR BREAKING CHANGE: Several plugins are not anymore loaded automatically as dependencies of other plugins. From now on, they need to be provided to the editor creator manually (via config.plugins). This list includes:

• The CloudServicesUploadAdapter plugin no longer loads CloudServices. Make sure to add CloudServices to the editor plugins when using the  CloudServicesUploadAdapter or EasyImage features.
• The EasyImage plugin no longer loads Image and ImageUpload. Make sure to add Image and ImageUpload to the editor plugins when using the EasyImage feature.
• The CKFinder plugin no longer loads CKFinderUploadAdapter. The CKFinderEditing plugin no longer loads ImageEditing and LinkEditing features. Make sure to add CKFinderUploadAdapter, Image, and Link features to the editor plugins when using the CKFinder feature.
• The Title plugin no longer loads Paragraph. Make sure to add Paragraph to the editor plugins when using the Title feature.
• The ListEditing plugin no longer loads Paragraph. Make sure to add Paragraph to the editor plugins when using the List feature.
• The LinkImageEditing plugin no longer loads ImageEditing. Make sure to add Image to the editor plugins when using the LinkImage feature.
• The LinkImageUI plugin no longer loads Image. Make sure to add Image to the editor plugins when using the LinkImage feature.

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Additional information

This going to be a huge PR but already reviewed. Let's keep merge notes here as I see that many of the changes might get lost.

Requires: ckeditor/ckeditor5-dev#683.