The link quick action should provide page and attachment suggestions

The link quick action of the BlockNote editor (typing /link and selecting "Link") opens the link modal directly, so even the most common case, linking to an existing wiki page, takes several steps. The image quick action already offers inline suggestions (XWIKI-24241); the link quick action should do the same, like the CKEditor link quick action mapped to [ does.

Expected behavior

• Selecting "Link" from the slash menu inserts [[, the start of a link in XWiki syntax, which opens the link suggestion menu. Typing [[ followed by some text opens the same menu.
• The text typed after [[ filters the suggestions, shown in two groups: wiki pages, then attachments. Up to 8 suggestions are shown: 5 pages and 3 attachments when there are enough matches of both kinds, the slots left free by one group being filled by the other (e.g. 6 pages and 2 attachments when only 2 attachments match).
• Selecting a page or an attachment inserts a link to it with a generated label (the page title, the attachment name), i.e. the link is saved without a label, e.g. [[doc:Space.Page]].
• Two actions are always available at the top of the menu:
  • Upload file... uploads a file (of any type) as an attachment of the edited page and inserts a link to it, with a generated label.
  • Link to... opens the link modal, to link to anything else (an external URL, an email address...) or to set the link options (display text, anchor, query string).