This removes the `grid` prefix from the gap-related properties of grid since they are now part of the box alignment specification. Former grid-gap* properties were aliased to the unprefixed properties to maintain compatibility.
The previously multi-column layout only `column-gap` property has been modified to apply to the Grid layout (and Flexbox in a following commit), moving the `mColumnGap` member variable from `nsStyleColumn` to `nsStylePosition`.
Notes:
* Bug 1398537 - support for percent values in column-gap for multi-column layout landed as part of Issue #1230. However, it was incomplete because it did not update `nsRuleNode` to allow transformation of percentage values for `column-gap`. This was consequently fixed as part of this commit.
* Bug 1456166 - this might not apply because we don't have that devtools test in UXP
* `nsRuleNode`, `nsCSSParser`, `Declaration`, and other related classes were merged into Stylo. These should be taken into consideration when porting patches from Mozilla.
Partially based on https://bugzilla.mozilla.org/show_bug.cgi?id=1398482
While we do fail a couple of tests, the other mainstream browsers also fail them and I think our implementation of tab-size is good enough to be unprefixed at this point. Having this patch also makes testing easier.
Since this is supported as an alias by Firefox and Edge for the same
reasons and we have websites using this to (attempt to) override the
system-provided styling with their own, leaving out the only supported
keyword we'd otherwise have (with -moz- prefix) but still stating
-webkit-.
TODO: unprefix this completely and make the vendor prefixes aliases.
