[feat] Mention

[Yxwww]

• [x] backspace should select mention if prev node is mention
• [x] delete all should trigger onChange callback with the correct mention state
  • use middleware
• [x] once user typed in an email address and followed by comma blur space etc, turn the email address to a chip/mention
  • [x] double click on NodeView will enable editing with suggestion
  • [x] add placeholder email invite icon
• [x] if autocomplete search is empty we don't show the dropdown
• [x] , space tab for converting new email into chip
• [x] click to edit feature for the mention chip
• [ ] handles gmail copy paste integration