Export more classes/types publicly

[jeffposnick]

R: @tropicadri

Fixes #1824, fixes #2952

The additional exports of TypeScript types that might be useful to some developers via the top-level index.ts is hopefully not controversial.

There are also two previously private, internal classes in the workbox-background-sync module that this PR makes public. Based on the initial issue request, there's a use for developers accessing these directly, but I've warned in the JSDoc that most developers shouldn't. You've touched the workbox-background-sync code most recently, @tropicadri, so I'll defer to whether you anticipate any problems with making those classes public.

[tropicadri]

The test check for the exported types and right now they are not matching:

1. [all] Window and SW packages
   should have top a level module for every export in index.mjs (and vise-versa):

   AssertionError: expected [ Array(4) ] to deeply equal [ 'BackgroundSyncPlugin', 'Queue' ]

   • expected - actual

   [
   "BackgroundSyncPlugin"
   "Queue"

   • "QueueStore"
   • "StorableRequest"
     ]

[jeffposnick]

Hmm... I remember when we added that restriction, and I guess it makes sense. I'll create a top-level module for each of those that re-exports the code in ./lib/. Technically the ./lib/ code isn't part of our public API, so I could probably just move the modules out of ./lib/, but I have a feeling there might be some people who are using them at their current location, so we'll have to save that until a future major release of Workbox.