chore: renamed downlevel-dts.ts to downlevel-dts.d.ts (#358)

With the upgrade of module resolution to Node16, we have to rename this
file to fix a compilation error.


---

By submitting this pull request, I confirm that my contribution is made
under the terms of the [Apache 2.0 license].

[Apache 2.0 license]: https://www.apache.org/licenses/LICENSE-2.0

Author: otaviomacedo