[css-tables] `overflow` override needs to handle `overflow-x` and `overflow-y`

For overflow: scroll and overflow: auto, https://www.w3.org/Style/css2-updates/REC-CSS2-20110607-errata.html#s.11.1.1b says

When used on table boxes, this value has the same meaning as 'visible'.

And equivalently from https://drafts.csswg.org/css-tables/#global-style-overrides

The overflow property on the table-root and table-wrapper box, when its value is not either visible, clip or hidden, is ignored and treated as if its value was visible.

However, overflow was a longhand on CSS2. css-overflow turned it into a shorthand, with the invariant that both overflow-x and overflow-y need to have consistent scrollability. So if one axis has a scrollable value (hidden, auto or scroll) and the other axis has a non-scrollable value (visible or clip), then the latter becomes scrollable (visible → auto, clip → hidden).

Then we need to preserve this invariant. If one axis computes to scroll or auto, then the other axis must also compute to a scrollable value. So if we make the former one behave as the non-scrollable visible, we have a scrollability mismatch. Browsers agree that then both axes must behave as visible.

Start with the CSS Tables global style overrides section and the linked CSS Overflow definition of overflow-x and overflow-y. Compare the stated scrollability invariant with the behavior described for table-root and table-wrapper boxes, including the browser behavior noted in the issue. Done means the specification clearly defines how both axes behave when one would otherwise become scrollable.