Docx reader: Support new table features.

* Column spans
* Row spans
  - The spec says that if the `val` attribute is ommitted, its value
    should be assumed to be `continue`, and that its values are
    restricted to {`restart`, `continue`}. If the value has any other
    value, I think it seems reasonable to default it to `continue`. It
    might cause problems if the spec is extended in the future by adding
    a third possible value, in which case this would probably give
    incorrect behaviour, and wouldn't error.
* Allow multiple header rows
* Include table description in simple caption
  - The table description element is like alt text for a table (along
    with the table caption element). It seems like we should include
    this somewhere, but I’m not 100% sure how – I’m pairing it with the
    simple caption for the moment. (Should it maybe go in the block
    caption instead?)
* Detect table captions
  - Check for caption paragraph style /and/ either the simple or
    complex table field. This means the caption detection fails for
    captions which don’t contain a field, as in an example doc I added
    as a test. However, I think it’s better to be too conservative: a
    missed table caption will still show up as a paragraph next to the
    table, whereas if I incorrectly classify something else as a table
    caption it could cause havoc by pairing it up with a table it’s
    not at all related to, or dropping it entirely.
* Update tests and add new ones

Partially fixes: #6316

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