Handling h1 headers

[f1ames]

Extracted from #1184.

As pointed out by @wwalc in #1184 (comment), pasting headers technically works (tags are correctly transformed), but since CKEditor supports h2 to h4 by default, h1 is transformed into paragraph which is quite confusing.

We need to think about some reasonable solution:

• Convert Word headings one level down so Word h1 will be transformed into h2 (again see Paste from Word - basic formatting support #1184 (comment) and Paste from Word - basic formatting support #1184 (comment)).
• If Paste from Word is enabled it could adjust the editor configuration so h1 tags are correctly transformed.
• Leave as is and educated users about this, showing the proper way to reconfigure editor so it works with h1.
• Any other idea which makes sense.

[Reinmar]

I think we need to finally resolve https://github.com/ckeditor/ckeditor5-heading/issues/98. And this doesn't touch MS Word – it touches all data, all pasted content. We should resolve that thing and then check if there's anything special that we need to do for MS Word.