Export additional interfaces for TSDocs

[jeffposnick]

There are some developer-facing interfaces that should be fully documented publicly, but don't show up in the generated HTML at https://developer.chrome.com/docs/workbox/reference/ because they're not exported from the top-level modules.

E.g. https://developer.chrome.com/docs/workbox/reference/workbox-expiration/#type-CacheExpiration references CacheExpirationConfig, but there is no context explaining what properties CacheExpirationConfig has.