This change adds back support for dispatching `scheduled` and `queue`
events directly. Miniflare 2 previously provided similar
`dispatchScheduled()` and `dispatchQueue()` methods, but these
implemented an inconsistent, non-standard API.

With `getFetcher()`, we're able to reuse magic proxy code to support
arbitrary Workers APIs. This is important for `queue()`, which
supports sending any structured serialisable as a message `body`.

`getFetcher()` also provides an idiomatic Miniflare API for dealing
with multiple workers, matching that provided by `getBindings()`.