Add YGGutter Enum

Summary:
This adds the YGGutter enum, used to choose between row/column gap variants (row-gap, column-gap, gap).

This used later in changes from https://github.com/facebook/yoga/pull/1116, in the APIs which deal with setting gap on style on yoga node.

Note the original PR called this `YGGap`, but this ending up leading to a couple public method signatures that could appear ambiguous:
1. `SetGap(YGGap gap, float gapLength)`: Enums like `YGAlign` are the vaues for an `align` prop. `YGGap` controls the variant of the gap (like `YGEdge` does for left/right/top/bottom variants). So the enum reads as if it is the `gapValue`, and it looks like we have two of the same parameter.
2. `SetGap(YGGap gapDirection, float gap)`: This is misleading, because the direction gaps flow is the cross-axis of flex-direction.
3. `GetGap(YGGap gap)`: `gap` is the variant, but looks like an out param.

The [CSS Box Alignment](https://www.w3.org/TR/css-align-3/#column-row-gap) spec refers to these gaps as "Gutters", which removes the ambiguity.

Changelog:
[General][Added] - Add YGGutter Enum

Reviewed By: yungsters

Differential Revision: D39922412

fbshipit-source-id: 4b0baf800fecb3d03560a4267c7fb4c4330fd39e

javascript/sources/YGEnums.js

@@ -53,6 +53,11 @@ const CONSTANTS = {
   FLEX_DIRECTION_ROW: 2,
   FLEX_DIRECTION_ROW_REVERSE: 3,
 
+  GUTTER_COUNT: 3,
+  GUTTER_COLUMN: 0,
+  GUTTER_ROW: 1,
+  GUTTER_ALL: 2,
+
   JUSTIFY_COUNT: 6,
   JUSTIFY_FLEX_START: 0,
   JUSTIFY_CENTER: 1,
@@ -147,6 +152,11 @@ export type Yoga$FlexDirection =
   | typeof CONSTANTS.FLEX_DIRECTION_ROW
   | typeof CONSTANTS.FLEX_DIRECTION_ROW_REVERSE;
 
+export type Yoga$Gutter =
+  | typeof CONSTANTS.GUTTER_COLUMN
+  | typeof CONSTANTS.GUTTER_ROW
+  | typeof CONSTANTS.GUTTER_ALL;
+
 export type Yoga$Justify =
   | typeof CONSTANTS.JUSTIFY_FLEX_START
   | typeof CONSTANTS.JUSTIFY_CENTER