[Miniflare 3] Add `getFetcher()` for dispatching `fetch`/`scheduled`/`queue` events

[mrbbot]

Hey! 👋 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().

[changeset-bot[bot]]

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Latest commit: 4a23fbafd0cc5e77e58b71e6a83f6f28c5467a49

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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