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/docusaurus.config.js

@@ -66,6 +66,7 @@ export default {
             position: 'left',
             label: 'Documentation',
           },
+          {to: '/playground', label: 'Playground', position: 'left'},
           {to: '/blog', label: 'Blog', position: 'left'},
           {
             href: 'https://github.com/facebook/yoga',
@@ -124,7 +125,11 @@ export default {
       },
       prism: {
         theme: prismThemes.github,
-        darkTheme: prismThemes.dracula,
+        darkTheme: prismThemes.oneDark,
+      },
+      colorMode: {
+        defaultMode: 'dark',
+        respectPrefersColorScheme: true,
       },
     }),
 };