support `library_note`

[Kitamado]

#help command library_note:

syntax "library_note"... [Batteries.Util.LibraryNote.commandLibrary_note___]
  \```
  library_note "some tag" /--
  ... some explanation ...
  -/
  \```
  creates a new "library note", which can then be cross-referenced using
  \```
  -- See note [some tag]
  \```
  in doc-comments.