DEV: use explicit default export in I18n shim (#22519)

Currently the I18n module shim return an object. Per AMD/loader.js,
the properties on the object becomes named exports of the module,
i.e. `import { t } from 'I18n';`.

However, this is not how we actually consume this module. We always
do `import I18n from 'I18n';`.

The returned object from the shim (`window.I18n`) does NOT have a
`default` property on it. This is only working because loader.js
has a `makeDefaultExport` feature that defaults to true, which we
are relying on to synthesize the default export for us.

That feature has been noted as undesirable and may some day be
deprecated. In Embroider, it specifically disables the feature in
loader.js.

https://github.com/embroider-build/embroider/issues/539

app/assets/javascripts/discourse/public/assets/scripts/module-shims.js

@@ -1,5 +1,5 @@
 define("I18n", ["exports"], function (exports) {
-  return I18n;
+  exports.default = I18n;
 });
 
 define("htmlbars-inline-precompile", ["exports"], function (exports) {