[css-display] Make `table-caption` and `table-cell` <display-outside> values

Splitting out from @MatsPalmgren's comment in https://github.com/w3c/csswg-drafts/issues/3771:

Random thought: maybe marker should be a display value rather than a position value.

It should be a new <display-outside> value, or an additional keyword, in that case. I think it would ruin this feature if you can't specify the marker's internal layout to be grid etc. Compare table-caption/cell, which has severely limited layout because you can't specify a <display-inside> value.

::marker, table-caption, <legend> and <summary> have a lot in common in this respect. They are in-flow by default, but not quite, since they aren't flowed normally together with other children in the layout context. Semantically, they're like a third variation of "flow" (besides in-flow, out-of-flow). I wonder if we should model them in the same way for that reason. Either through <display-outside> or a new property. E.g.

<display-outside> = block | inline | run-in | marker | caption | cell | legend | summary

This would add a lot more power to authors to control the layout without having to add wrapper elements. E.g. <caption> would have display: caption flow-root by default, but you could also specify <caption style="display: caption grid"> to display its children using Grid layout.
legend | summary allows an arbitrary child to be displayed as the rendered legend/summary in a fieldset/details element.

(FYI, It would be trivial to support table captions/cells with internal grid/flex/table layout in Gecko. The only thing missing is some way to specify it in CSS.)

It would be trivial to make table-caption a <display-outside> value in the spec, too. Should we do it? (By which I mean, does anyone actually want to implement it?)

Start with the CSS Display 3 definition and the linked discussion in issue #3771. Review how table-caption and table-cell are currently specified, then determine the wording and interoperability concerns needed for a Working Group resolution. Done means the proposal has consensus and the relevant specification text is updated.