biblio notes

[opoudjis]

Currently bibitem/note is restricted to Metanorma inline text:

## A note associated with the bibliographic item
biblionote = element note {
  ## The class of the note associated with the bibliographic item.
  ## May be used to differentiate rendering of notes in bibliographies
  attribute type { text }?,
  ## The content of the note
  LocalizedMarkedUpString
}

But we cannot prevent notes being one or more paragraphs, and indeed, that is already happening when I interleave notes into Metanorma bibitems, to indicate their publishing status. bibitem/note needs to be like bibitem/abstract, and consist of at least one paragraph:

## Abstract of bibliographic item
bibabstract = element abstract { 
  LocalizedStringAttributes,
  ( 
    ## Multiple blocks of content
    BasicBlockNoId+ |
    ## Single block of content
    TextElement+
  )
}