client: Change target from ES6 to ES2016 (ES7)

As all mainstream browsers passing the current MediaWiki feature checks support ES2016, raise the default wikimedia/client configuration to allow ES2016 syntax and methods.

Bug: T381537

client.json

@@ -1,3 +1,3 @@
 {
-	"extends": "./client/es6"
+	"extends": "./client/es2016"
 }

client/es2016.json

@@ -0,0 +1,7 @@
+{
+	"extends": [
+		"./common",
+		"../language/es2016",
+		"../vue/es2016"
+	]
+}

readme.md

@@ -12,24 +12,24 @@ Then, [configure ESLint](https://eslint.org/docs/user-guide/configuring) with on
 Below are some potential recommended uses:
 
 ### A typical front-end project
-This config allows ES6 code and browser native functions. It will complain about ES2016+ code and language features.
+This config allows ES2016 code and browser native functions. It will complain about ES2017+ code and language features.
 It also automatically applies the Vue plugin and Vue-specific rules to `.vue` files.
 
 `.eslintrc.json`:
 ```json
 {
-	"extends": "wikimedia/client/es6"
+	"extends": "wikimedia/client/es2016"
 }
 ```
 If you want to only allow ES5 code and browser native functions, you can use `wikimedia/client/es5` instead.
 
 #### jQuery
-This config adds the jQuery `$` global, and additional rules preventing the use of jQuery features which are deprecated, have performance issues, or have simple ES6 alternatives.
+This config adds the jQuery `$` global, and additional rules preventing the use of jQuery features which are deprecated, have performance issues, or have simple ES2016 alternatives.
 `.eslintrc.json`:
 ```json
 {
 	"extends": [
-		"wikimedia/client/es6",
+		"wikimedia/client/es2016",
 		"wikimedia/jquery"
 	]
 }
@@ -41,7 +41,7 @@ Code that runs in MediaWiki can use this config. It enforces rules that are spec
 ```json
 {
 	"extends": [
-		"wikimedia/client/es6",
+		"wikimedia/client/es2016",
 		"wikimedia/mediawiki"
 	]
 }

vue/es2016.json

@@ -0,0 +1,3 @@
+{
+	"extends": "../vue3/es2016"
+}

vue3/es2016.js

@@ -0,0 +1,12 @@
+'use strict';
+
+/* eslint-disable quote-props, quotes */
+module.exports = {
+	"overrides": [ {
+		"files": [ "**/*.vue" ],
+		"extends": [
+			"../vue3/common",
+			"../language/es2016"
+		]
+	} ]
+};