Bump ecmaVersion and update environment for ESLint

ESLint doesn't recognize `async function` with `ecmaVersion: 6`.
Make the environment the same as in jellyfin-web.

Author: dmitrylyzo

.eslintrc.yml

@@ -1,6 +1,8 @@
 env:
   es6: true
   browser: true
+  es2017: true
+  es2020: true
 
 extends:
   - "eslint:recommended"
@@ -18,7 +20,7 @@ rules:
   promise/no-return-wrap: ["warn"]
 
 parserOptions:
-  ecmaVersion: 6
+  ecmaVersion: 2020
   sourceType: module
 
 overrides: