feat: allow additional handlers to be exported by adapter-cloudflare

[thomasfosterau]

closes #10117
closes #1712

Cloudflare Workers allows for handlers to be exported from a worker as methods on an object exported as a default export. Currently, adapter-cloudflare-workers exports a fetch handler method, but there is no way to add other handler methods, such as a scheduled or queue handler. This functionality was requested just over a year ago in #10496.

This PR adds a configuration option for @sveltejs/adapter-cloudflare to provide a file whose exports get added to the default option exported by the worker the adapter generates. The fetch handler does not get overridden.

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Latest commit: c9cd1de

The changes in this PR will be included in the next version bump.

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@sveltejs/adapter-cloudflare	Minor

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[thomasfosterau]

This is no longer a draft as all test, lints and checks should now be passing. This PR replaces #13207 as the two Cloudflare adapters have now been merged into one.

The approach here is the same as in #13207: the Cloudflare adapter optionally takes a path to a file which exports anything that can be exported from a Cloudflare Worker, except for the fetch property of the default export.

I have intentionally not followed what the Node adapter does (generate two files: one which exports the SvelteKit handler, and one which runs that handler as a server, allowing users to optionally import the handler and run it in their own server) as I don’t think there is a strong reason to do it for this adapter, given a Cloudflare Worker is just an export handler and not a server itself. Correct me if I’m wrong, but most of the use cases of a custom fetch handler can be realised inside SvelteKit itself, specifically inside the handle server hook or reroute. Any advanced use cases can import the fetch handler from the generated worker if necessary.

[thomasfosterau]

For reference, the following handlers will work as properties on the default export, alongside fetch:

• alarm
• email
• queue
• scheduled
• tail

The following functionality is available as classes which can be exported as named exports from the file:

• Durable Objects
• RPC/Service bindings
• Workflows