Annotate the parse tree of lists

[pmario]

1. The full row marker of each item (e.g. *#> in mixed nesting, or
   which of the description-list forms ;/: produced an item). The
   tag nesting alone cannot always reproduce the exact marker string.
   Annotation: rowMarker (the matched marker text) on each item.

[
    {
        "type": "element",
        "tag": "ul",
        "children": [
            {
                "type": "element",
                "tag": "li",
                "children": [
                    {
                        "type": "text",
                        "text": "level 1",
                        "start": 2,
                        "end": 9
                    }
                ],
                "start": 0,
                "end": 9,
                "rowMarker": "*"     <-- New marker for easy serialising
            }
        ],
        "start": 0,
        "end": 9,
        "rule": "list",      <-- All lists have rule: "list"
        "blockPosition": true
    }
]