Twitch already has it. I cast to my Chromecast all the time.

If you have a Chromecast, you should be able to cast to it. I see you mentioned "tv", so I assume that you want to cast to a smart TV. Then, your TV needs to either have a chromecast built-in or it needs to have a Twitch TV app. For example, my TV doesn't have a Twitch app, only a Netflix, Hulu, and Youtube app. If I want to cast to one of those 3, I can cast directly to the TV. However, if I want to cast a Twitch stream, I must cast to my Chromecast that's plugged into the TV.

My TV is a few year old Samsung TV. It has an app store, but there's no Twitch app on it. I don't know if more recent TVs have a Twitch app that can be installed directly.