Add opt-in behavior for custom transforms to support bundles

[rbuckton]

As an alternative approach to #29871, this PR addresses a backwards-compatibility issue when we added support for transforming Bundle nodes. Custom transformers have the type TransformerFactory<SourceFile>, as they only expect to receive source files. However, we assign them to an array of TransformerFactory<Bundle | SourceFile>[] and may inadvertently pass a Bundle to a transformer that is not able to support it.

To address this, I've added an opt-in mechanism where custom transform authors can return a CustomTransformer object or the existing Transformer<SourceFile> callback. Custom transformers are then wrapped in a callback that invokes the transformation based on whether it is a function (and correctly wraps it in a call to chainBundle), or an object (in which case the appropriate transformSourceFile or transformBundle methods are called).

Fixes #28310, #24459
Replaces #29871

[weswigham]

Good, but that union type looks like a typo 😉