Support for basic list indentation when pasting from Word

[f1ames]

Follow up of #2482.

As support for pasting flat lists from Word is (almost) ready (see ckeditor/ckeditor5-paste-from-office#7), the next step for handling pasted lists should be correctly handling list items indentation. Since CKEditor 5 only supports rather basic indentation it might not be necessary/needed to go with all the cases (e.g. when difference between indented adjacent items is greater than one level), but basic support for indentation should be provided at some point.

There are two mechanisms which decides how list items will be indented in this case - paste from office plugin transforming pasted content and lists conversion mechanism (list plugin). The good approach for this ticket would be to make paste from office plugin transform indentation 1:1 (without simplifying/normalizing nested levels), which means providing structure which may not be fully supported by list converters. Then lists conversion will simplify the structure if needed (hopefully without any invalid/strange results).
If more complex indentation model will be needed at some point in future it will require only changes in list plugin conversions without adjusting paste-from-office as it will be already providing proper structure.

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