feat(fetch-router): add static-middleware

[markdalgleish]

This is still a work in progress, so I'm just sharing for some early feedback.

[mjackson]

This is looking great. Very clean! I love how organized you're keeping the code.

There are just a few small ways I think we could re-arrange things that would make it a little more ergonomic IMO. Hopefully the feedback I left helps.

[mjackson]

What if we unwound this a bit so we didn't have so much wrapped up inside the handler?

For example, we could do something like:

let resolveFile = createFsFileResolver(root, pathResolver)
let file = resolveFile(context)

if (file == null) {
  return next()
}

return sendFile(file, context)

This way we wouldn't have to generate a 404 Response in the handler that we just throw away at this level.

[mjackson]

This is where staticFiles really shines, IMO. In the "global" middleware case.

Trying to re-use staticFiles as an inline middleware on a specific route feels a little awkward because we have to pass the param from the route to the middleware somehow. I think I'd prefer to have a more direct approach in a route handler, something like:

router.get('/assets/*path', (context) => {
  let file = findFile('./assets', context.params.path)
  if (file != null) return sendFile(file, context) // sendFile could handle ETag, Ranges, etc.
  return new Response('Not Found', { status: 404 })
})