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
Moonchild
MeladJM
erixreyes
Francis Dominic Fajardo
FranklinDM
yami
Matt A. Tobin