Make standard CSS properties only belong to one group

[wbamberg]

This is a partial resolution to #239.

It updates data for standard CSS properties to make sure that they only belong to one "group", that being the most recent spec that defines them. This should be enough to make sidebars more coherent for pages like column-gap.

This is a little dodgy, I suppose, since the "most recent spec" is generally less mature than the others (e.g. break-after is still defined in CSS Regions, with the note:

This section is also defined in [CSS3-BREAK]. If that specification moves to last call with the region values, the section here can be replaced by a reference.

(https://drafts.csswg.org/css-regions-1/#region-flow-break)

But I guess that realistically, it's safe to consider that break-after is now defined in the CSS Fragmentation spec.

This PR does not update data for various nonstandard properties:

box-flex-group
box-ordinal-group
box-flex
-moz-appearance
box-lines
box-orient
box-pack
box-direction
box-align

@rachelandrew , please let me know if this looks good to you.

[rachelandrew]

Looks good and I agree re break-after, that should be considered part of fragmentation.

[chrisdavidmills]

This approach looks good to me. r+

I didn't check the whole file, but I did look over your changes and they look good. If we missed any instances, we can always iterate later on.