(Chore) Update Angular example code

[csmeyns]

This change updates the Angular example code, as follows:

• Removes the angular6 folder with example code, which was paired with an older tutorial that has itself been removed.
• Adds the angular-chat-app folder with example code, which belongs to the tutorial: How to build a chat into an Angular app with TalkJS. The code itself initially lived on Aswin Rajeev's personal repo (Aswin is the tutorial author). This change brings it into talkjs-examples.

[csmeyns]

Thanks! Yes will update the links in the tutorial 👌