[upstream] Allow copying semantic structure from Word without non-semantic styling to prevent insertion of extraneous non-breaking spaces

Thanks for the detailed bug report! 🙏 I have two more questions to be able to raise this with the CKEditor team:

1. Can you reproduce this at https://ckeditor.com/ckeditor-5/demo/feature-rich/?
2. Did you check if https://github.com/ckeditor/ckeditor5/issues has an issue for this already?

@Wim Leers: in answer to your questions:

1. I am unable to tell whether I can reproduce it on the CKEditor feature-rich editor page, as that page does not provide a Source button. However, I am able to reproduce it on the CKEditor demo page.

2. There was an existing issue [Safari] Non-breaking space present in a model after view upcasting in some specific cases #2522. However, it was different enough that I filed a separate issue Paste from Word introduces non-breaking spaces #15488.

Setting to Active on the basis that I provided the info, as I don't see a Postponed (waiting on upstream) option.

I renamed the issue to "Allow copying structure from Word without styling" because, according to a maintainer comment concerning non-breaking spaces in Can't use non-breaking-space in the editor from one of the maintainers, CKEditor5, CKEditor5 manages non-breaking spaces and places them where it "thinks" they belong.

The title reads like a feature request. However, because CKEditor5 inserts unwanted non-breaking spaces into some documents copied from Word, lack of this feature manifests as a bug. Either non-breaking spaces in CKEditor5 need to be fixed (see #1669) or it has to be possible to strip Word styling on paste so that CKEditor 5 no longer inserts extraneous non-breaking spaces.

If/when this is implemented, I would want this to be an administrator configuration, not the choice of individual editors. That is, if I as administrator configure the site that pasting from Word into the CKEditor WYSIWYG is to strip non-semantic styling, then editor pastes would always strip non-semantic styling from the code as well as any span tags which were now empty because of the stripping of that styling.