Add async and charset attributes to the twitter external library script

Problem/Motivation

As per Twitter embed documentation, it's recommended to add the async and charset attributes while loading the twitter external library as follows:
<script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script>

Steps to reproduce

N/A

Proposed resolution

Add these attributes in media_entity_twitter.libraries.yml

Remaining tasks

To be reviewed

User interface changes

N/A

API changes

N/A

Data model changes

N/A