Provide API for service worker caching strategies

[dillonkearns]

It would be nice to create, essentially, a type-safe wrapper for the Workbox API to give users control over the service worker caching strategy.

It should use PagePaths and ImagePaths, specifically, to make it as safe as possible.

It might be nice to have a function that passes in each static route or image, and then you can return a cache strategy.

cacheStrategy : PagePath Pages.PathKey -> CacheStrategy
cacheStrategy path =
  if  path |> Directory.includes Pages.pages.blog.directory then
    Precache
  else
    NetworkFirst

That Elm code can be used to just create a hardcoded list of routes and what caching strategy should be used. Although, what about forward-compatibility, since service workers could be out of date? Ideally, elm-pages would refresh the service worker when it's out of date, but it's something to consider.