Prevent our 'no-restricted-properties' from clobbering eslint-config-wikimedia

Our eslintrc extends from 'wikimedia/client' which includes a
'no-restricted-properties' ruleset from the 'not-es5.js' file [1].

However, we were also including our own 'no-restricted-properties'
rules.

ESLint handle this duplication by clobbering instead of merging
so eslint-config-wikimedia's no-restricted-properties where not taking
effect and we were losing out on some guards against using es6.

This commit corrects that and makes both no-restricted-properties
rulesets merge instead of clobber as already done in MobileFrontend [2]

[1] 07320f16ae/language/not-es5.js (L5)
[2] https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/.eslintshared.js

Bug: T239269
Change-Id: Ibc2c144be51719d71a4c1d5828486253a5d4bf5d

.eslintrc.js

@@ -1,4 +1,10 @@
-{
+// "no-restricted-properties" rules are not properly merged when just using "extends".
+// Therefore we have to have this file which calls a custom merge function.
+// The merge function calls Object.assign with special handling for configuration such as
+// `no-restricted-properties` and `no-restricted-syntax` which are array based - ensuring the two
+// values being merged are concatenated.
+const merge = require( 'eslint-config-wikimedia/language/merge.js' );
+const config = {
 	"root": true,
 	"extends": [
 		"wikimedia/client",
@@ -40,4 +46,9 @@
 		"no-use-before-define": "off",
 		"no-underscore-dangle": "off"
 	}
-}
+};
+
+module.exports = Object.assign(
+	config,
+	merge( config, require( 'eslint-config-wikimedia/language/not-es5.js' ) )
+);

Gruntfile.js

@@ -1,4 +1,5 @@
 /* eslint-env node */
+/* eslint-disable no-restricted-properties */
 module.exports = function ( grunt ) {
 	var conf = grunt.file.readJSON( 'skin.json' );
 

tests/qunit/.eslintrc.json

@@ -1,7 +1,7 @@
 {
 	"extends": [
 		"wikimedia/qunit",
-		"../../.eslintrc.json"
+		"../../.eslintrc.js"
 	],
 	"rules": {
 		"no-restricted-properties": "off",