(This should stay draft and not merge until we have some consensus on the ecosystem-spanning parts of the migration.)

This is an exploration to see how easy it is to make the ember-provided decorators work under both legacy decorators and stage3 decorators.

(I don't think it's hard!)

The first commit here implements `@tracked`. I tested it in an application with the configuration:

```js
 const app = new EmberApp(defaults, {
    'ember-cli-babel': {
      disableDecoratorTransforms: true,
    },
    babel: {
      plugins: [
        [
          '@babel/plugin-proposal-decorators',
          {
            version: '2023-05',
          },
        ],
      ],
    },
  });
```

With those settings, `@tracked foo` becomes a runtime error and will tell you to use `@tracked accessor foo` instead, and when you do it all works correctly.