Improve keyboard navigation around inline widgets

[jswiderski]

📝 Provide a description of the new feature

What is the expected behavior of the proposed feature?

It is either hard or impossible to get to inline widget using keyboard. If you have 3 lines: 1.text 2. Inline Widget 2.text then in Firefox it will not be possible to get to widget using arrows while in Chrome it is not possible to get to widget using up/down arrows but left/right arrows work ok. 

Inline widget used for testing is Placeholder plugin from our documentation - https://ckeditor.com/docs/ckeditor5/latest/framework/guides/tutorials/implementing-an-inline-widget.html#full-source-code

Firefox (first left/right, next up/down arrow test):

Chrome (first left/right, next up/down arrow test):

NOTE: We have 3 other issues related to navigation or selection around widgets: #6741, #4604, #8333

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[CKEditorBot]

There has been no activity on this issue for the past year. We've marked it as stale and will close it in 30 days. We understand it may still be relevant, so if you're interested in the solution, leave a comment or reaction under this issue.

[CKEditorBot]

We've closed your issue due to inactivity over the last year. We understand that the issue may still be relevant. If so, feel free to open a new one (and link this issue to it).