build(eslint): enable usage of the "spread" operator (dequelabs#1034)

The patch updates our ESLint configs and instructs ESLint to enable ECMA v9 features (including the spread `...` operator).

Additionally, a random usage of `Object.assign()` has been replaced by the spread operator to ensure this is working as expected.

.eslintrc

@@ -1,5 +1,8 @@
 {
   "extends": ["prettier"],
+  "parserOptions":{
+    "ecmaVersion": 9
+  },
   "env": {
     "node": true
   },

doc/examples/chrome-debugging-protocol/.eslintrc

@@ -3,6 +3,6 @@
     "es6": true
   },
   "parserOptions": {
-    "ecmaVersion": 8
+    "ecmaVersion": 9
   }
 }

doc/examples/puppeteer/.eslintrc

@@ -3,6 +3,6 @@
     "es6": true
   },
   "parserOptions": {
-    "ecmaVersion": 8
+    "ecmaVersion": 9
   }
 }

lib/core/base/audit.js

@@ -16,14 +16,7 @@ function getDefaultConfiguration(audit) {
 	config.reporter = config.reporter || null;
 	config.rules = config.rules || [];
 	config.checks = config.checks || [];
-	config.data = Object.assign(
-		{
-			checks: {},
-			rules: {}
-		},
-		config.data
-	);
-
+	config.data = { checks: {}, rules: {}, ...config.data };
 	return config;
 }