Mention: do not confirm selection on "space"

[fredck]

📝 Provide a description of the improvement

Currently, it is cumbersome to simply type text that uses the mention marker but is not in the mention list, in some cases.

For example, suppose this:

• My app loads just the last 500 issue numbers of CKEditor 5 in the mention list.
• Let's say we have 1200 issue is total -> therefore issues from 700 to 1200 are loaded.
• My app uses hash (#) as the marker to mention issues.
• I want to mention issue 1. I type #1.
  • At this point, the mention list is shown. As the list is filtered by what I typed, I see issues 1000, 1001, 1002... with issue 1000 pre-selected.
• I want to continuer typing after #1, so I hit space.
  • Issues: my typed #1 is replaced with #1000. That's because space was used to confirm my selection in the list, which is clearly not what I wanted.

This seems to be caused by this line, which tells me that even tab could be used to confirm my select 🤔 
https://github.com/ckeditor/ckeditor5-mention/blob/9a514ccb0dc8b595100fd4459a82fa67cc329d1d/src/mentionui.js#L124

I propose to allow confirming the selected item by using the enter key, exclusively.

📃 Other details

• CKEditor version: v17.0.0
• Installed CKEditor plugins: mention

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[fredck]

In some senses this is a dup of #4665, with the difference that I find it over-engineering to make this configurable. I'm my POV, allowing space and tab for that purpose was simply a bad design decision to be stripped out.