Add private runloop backburner re-exports

`@ember/test-helpers` uses these as they exist in the DT and preview
types, so to avoid breaking that, re-supply them here. They should not
be relied upon long-term, but this allows us to migrate the library's
dependency chain distinctly from the release of these types.

Author: chriskrycho

packages/@ember/runloop/-private/backburner.ts

@@ -0,0 +1,15 @@
+// This exists solely to provide a compatibility shim for `ember-test-helpers`
+// as part of the transition off of DefinitelyTyped. Long-term, this should be
+// structured differently:
+//
+// 1. `backburner.js` should provide actual public types for things which are
+//    actually part of its public contract: these are currently used in key
+//    areas as "intimate" API.
+// 2. `ember-test-helpers` should use the types from `backburner.js` directly,
+//    rather than relying on Ember's re-export.
+
+import type Backburner from 'backburner.js';
+export type DebugInfo = ReturnType<Backburner['getDebugInfo']>;
+
+export { IQueueItem as QueueItem } from 'backburner.js/dist/backburner/interfaces';
+export { DeferredActionQueues } from 'backburner.js';