fix(bundle-source): Export types properly

[kriskowal]

Description

Using dynamic import in bundleSource made it possible to use bundleSource for one format and not entrain all the tooling for other formats, like Rollup for getExport and Babel for endoZipBase64. But, it broke the flow of type information. This change proposed by @michaelfig rectifies the internal and external types.

Security Considerations

None.

Scaling Considerations

None.

Documentation Considerations

None.

Testing Considerations

This PR does not adequately test the exported type signature. It would pair well with a change that relies on the exported types.

Compatibility Considerations

None.

Upgrade Considerations

None.

• [ ] Includes *BREAKING*: in the commit message with migration instructions for any breaking change.
• [ ] Updates NEWS.md for user-facing changes.

[kriskowal]

I’ve tacked on a commit for @dckc to relax the bundle cache option to be actually optional.