Playground: Pull out the core so it can be reused

[ajhager]

It would be nice if there was a more generic library and a tool that helped you create custom builds of the playground with custom packages built in. For instance, on the blog I mention how neat it would be to have a custom SVGo code sketch that made use of the playground.

In the same vein, the playground should probably be its own repo, with only builds checked into the website.

See this repo to follow these explorations.

[dmitshur]

If #26 is implemented, I think it would solve this issue more generally. Do you have any comments on that @ajhager?