[Feature Request] Use anonymous (readonly) chat if `TWITCH_BOT_USERNAME` is not provided

[DmitryScaletta]

Motivation

1. It's pretty hard for some not experienced users to configure TWITCH_BOT_USERNAME and TWITCH_BOT_OAUTH
2. Some people might not want to use all the chat commands, or don't want to see NOALBS messages in the chat
3. After some time (a few months or so) the token will expire

Implementation

It's possible to connect anonymously to the chat:
TWITCH_BOT_USERNAME: justinfan${randomSuffix}, examples: justinfan65742, justinfan02332
TWITCH_BOT_OAUTH: empty string

How to generate bot username: https://github.com/twurple/twurple/blob/7a18335c1aba42a49040a21bc15b8243f49d8282/packages/chat/src/ChatClient.ts#L1379-L1384