Bogus <br /> handling during typing

[scofalik]

Suggested merge commit message (convention)

Fix: Bogus <br /> element inserted by a browser at the end of an element is now correctly handled. Closes ckeditor/ckeditor5#1083.

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Additional information

This is not a complete solution. I don't like two things:

• Inserting @ in place of non-text elements. I tested it manually and it looks like everything works well but I am a little skeptical because a user may type @ by themselves and I am afraid it may mess up the diff. Still, I tried putting <br /> between typed @s and it worked 🤷‍♂️ .
• Only <softBreak> "inline" element is now supported. I wanted to do it in a future-proof way but I don't know how you can differentiate that given elements are "inline" elements (like <softBreak>). Basically, when we will introduce a new inline element, like smileys or maybe placeholders, we again will have a problem in isSafeForTextMutation function. Maybe we could introduce isInline flag in SchemaItemDefinition?

[coveralls]

Coverage remained the same at 100.0% when pulling fb4e3c1 on t/ckeditor5/1083 into 039f5d3 on master.

[Reinmar]

OK, I tested it quite thoroughly and will merge it. However, let's have a confirmation – @pomek, could you check it too? Try being creative :D And remember to check it on all browsers.

[pomek]

I'll do all the best what I can in order to check this out :D

[Reinmar]

I reported ckeditor/ckeditor5#1096 as a followup.