Add experimental snapshot field (not published) (web-platform-dx#625)

Author: Elchi3

feature-group-definitions/array-at.yml

@@ -1,5 +1,6 @@
 name: Array at()
 spec: https://tc39.es/ecma262/multipage/indexed-collections.html#sec-array.prototype.at
+snapshot: ecmascript-2022
 status:
   baseline: low
   baseline_low_date: 2022-03-14

feature-group-definitions/array-flat.yml

@@ -1,6 +1,7 @@
 name: Array flat() and flatMap()
 caniuse: array-flat
 spec: https://tc39.es/ecma262/multipage/indexed-collections.html#sec-array.prototype.flat
+snapshot: ecmascript-2019
 status:
   baseline: high
   baseline_low_date: 2020-01-15

feature-group-definitions/async-await.yml

@@ -1,6 +1,7 @@
 name: Async functions
 spec: https://tc39.es/ecma262/multipage/ecmascript-language-functions-and-classes.html#sec-async-function-definitions
 caniuse: async-functions
+snapshot: ecmascript-2017
 status:
   baseline: high
   baseline_low_date: 2017-04-05

feature-group-definitions/bigint.yml

@@ -1,6 +1,7 @@
 name: BigInt
 spec: https://tc39.es/ecma262/multipage/numbers-and-dates.html#sec-bigint-objects
 caniuse: bigint
+snapshot: ecmascript-2020
 status:
   baseline: high
   baseline_low_date: 2020-09-16

feature-group-definitions/class-syntax.yml

@@ -1,6 +1,7 @@
 name: Classes
 caniuse: es6-class
 spec: https://tc39.es/ecma262/multipage/ecmascript-language-functions-and-classes.html#sec-class-definitions
+snapshot: ecmascript-2015
 status:
   baseline: high
   baseline_low_date: 2017-03-27

index.ts

@@ -13,7 +13,8 @@ const monthsFromBaselineLowToHigh = 30;
 // Some FeatureData keys aren't (and may never) be ready for publishing.
 // They're not part of the public schema (yet).
 const omittables = [
-    "description"
+    "description",
+    "snapshot"
 ]
 
 function scrub(data: FeatureData) {