using `js-module-formats` to detect source type

In the transition period to ES modules, we might have mixes of modules formats in our source base, and having a transpiling process that can detect and apply the proper workflow will always come handy. This PR relies on `js-module-formats` (which now supports `steal` https://github.com/yahoo/js-module-formats/releases/tag/v0.1.2) to detect any source format, and falls back to `es6` when detection fails.

Additionally, I intentionally included the branch of PR #1 to validate that tests are still passing.