[TASK] Improved ESLint config for Backend JavaScript (#476)

With this configuration, ESLint will no longer complain about the `TYPO3` and `$` global, `define()` calls, etc. It will also allow the use of `const`.

Resolves: #475

.eslintrc.json

@@ -5,6 +5,16 @@
 		"plugin:prettier/recommended"
 	],
 	"env": {
-		"browser": true
+		"browser": true,
+		"node": true,
+		"es6": true,
+		"amd": true,
+		"jquery": true
+	},
+	"globals": {
+		"TYPO3": "readonly"
+	},
+	"parserOptions": {
+		"ecmaVersion": 8
 	}
 }