Ckeditor5: Open utils used by the mention feature

[Reinmar]

The initial plan for https://github.com/ckeditor/ckeditor5-mention was that it's going to be a concrete implementation of "autocompletion" while we still planned to work on opening the tools needs to build your own autocompletion features.

The mention feature is sufficient for things like inserting @mentions or #hastags. It also implements specific behaviour of the mention item in the editor (e.g. removing the mention if someone modifies its text). It's fairly configurable, however, it does not allow some customizations:

• What kind of markup is inserted into the editor. You can only configure how a mention is output in the data, but you cannot force the mention feature to insert e.g. a placeholder widget.
• Where the autocompletion panel is shown.
• What happens during autocompleting (how the panel behaves, what's in the editor).

Example features which will be possible to develop with those tools:

• completing placeholders (see CKEditor 4's https://ckeditor.com/docs/ckeditor4/latest/examples/autocomplete.html)
• completing emojis
• Slack-like autocompletion panel (above/below the text area)

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[jodator]

A case for autocomplete - almost like Mention plugin but with custom "slash commands":

• JIRA slash commands: https://confluence.atlassian.com/hipchatdc3/keyboard-shortcuts-and-slash-commands-966656108.html
• Notion slash commands: https://www.notion.so/Learn-the-shortcuts-66e28cec810548c3a4061513126766b0#fd1e1bf866f74e1aa95ff8511e1afe25