Colocation babel plugin drops class decorators

With an input like

```js
import hbs from 'ember-cli-htmlbars-inline-precompile';
const __COLOCATED_TEMPLATE__ = hbs`{{yield}}`;
export default @dec class {}
```

The class decorator `@dec` gets dropped:

```js
import hbs from 'ember-cli-htmlbars-inline-precompile';
const __COLOCATED_TEMPLATE__ = hbs`{{yield}}`;
export default Ember._setComponentTemplate(__COLOCATED_TEMPLATE__, class {});
```

See example https://astexplorer.net/#/gist/4fb9360091ca6422ca0f9e4ee8ef9505/90046a397d94a8ccf8e97c17fd4f2d0cb867a129

I think all that is required to fix is to pass `defaultExportDeclaration.decorators` as the fourth argument to `t.classExpression()`.

Start with the colocation Babel transform that handles the default export, using the provided AST Explorer example to reproduce the dropped class decorator. Preserve the decorator in the transformed class expression and verify that the generated output retains @dec.

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