API: Make public internal chainBundle

[cevek]

TypeScript Version: 3.2.0-dev.201xxxxx

Currently we can't work with bundles in transformers, but built-in transformers use chainBundle to iterate over sourceFiles inside them

return chainBundle(transformSourceFile);

So, we need something similar or opened chainBundle to use in the custom transformers

[SalvatorePreviti]

+1, it would be really helpful.