[playground] Add button to create gist from current ytt playground session

**Describe the problem/challenge you have**
When I am creating an example in the ytt playground, and I want to turn the example into a gist, I have to open a gist and copy paste each file over.

**Describe the solution you'd like**
I would like a button in the playground that copies all files in my playground session into a single new gist.
If I was looking that the [playground example demo](https://carvel.dev/ytt/#example:example-demo), and I clicked the "Copy to gist" button, I would expect a [gist window](https://gist.github.com/gcheadle-vmware/ebe7ce972fce9dd3fa59e9e42dc7fc3c) to pop up in a new tab that I would be able to name and create the gist.


---
Vote on this request

This is an invitation to the community to vote on issues, to help us prioritize our backlog. Use the "smiley face" up to the right of this comment to vote.

👍 "I would like to see this addressed as soon as possible"
👎 "There are other more important things to focus on right now"

We are also happy to receive and review Pull Requests if you want to help working on this issue.