Replace Playground with JSX Editor (#1500)

Summary:
Pull Request resolved: https://github.com/facebook/yoga/pull/1500

Inspired by the frequent usage of Expo snacks to run RN, to repro Yoga issues, this replaces the Playground port with a new ground up Playground UI. This UI right now is pretty simple, with a JSX editor which creates a Yoga tree, which is then rendered using the WebAssembly variant of Yoga.

There are a lot of ways we can continue to improve this, but this merges the foundation. Subjectively, I find this more useful as a tool to play with Yoga behavior than the GUI.

This also replaces some of the bits of the homepage, and adds a playground entrypoint (though it's pretty identical to the one I've been testing on the home page).

Reviewed By: yungsters

Differential Revision: D51963201

fbshipit-source-id: 1265cb1784151b685686e189d47ecd42cbacdf8f

website-next/static/img/copy.svg

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website-next/static/img/link.svg

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