[css-forms-1] Add fieldset and legend

[lukewarlow]

See #11983

[nt1m]

I think we should spend some time thinking about what kind of improvements we want to give to fieldset/legend, and whether it makes sense for them to follow the appearance: base pattern.

e.g. for fieldset, if the main thing we want to change is the border to also be currentColor to match the rest, it might just be possible to change the existing border: 2px groove threedface to border: 1px solid currentColor directly everywhere with negligible/no web compat breakage. We've changed the border-color before for things like table or hr successfully, so I don't think this is infeasible.

For legend, I've heard valid complaints about the layout being hardcoded and not overridable, and there was some attempt from @zcorpan to previously address it using appearance: none. I would consider filing an issue against the css-forms-1 to explore this further. Maybe appearance: base would also make sense here, but I wouldn't rush into it without having the discussion first.

[lukewarlow]

#11983

[zcorpan]

See whatwg/html#3912

The conclusion in whatwg/html#4035 (comment) was that appearance is not the right mechanism to control the magic of fieldset rendering.

[zcorpan]

The rendering rules are defined here https://html.spec.whatwg.org/#the-fieldset-and-legend-elements (this could probably be moved to a CSS spec though).