feat: support AWS via SST in adapter-auto

[fwang]

This PR adds a check for AWS via SST to adapter-auto. If it succeeds, it uses svelte-kit-sst, which we maintain. This package is not automatically added as a dependency of adapter-auto, users need to install it first.

The check uses the SST environment variable to check if we are building using SST.

I tested this using patch-package on one of my SWA projects and it seemed to work.

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@sveltejs/adapter-auto	Minor

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[benmccann]

I don't think we'd want to add any docs here. We've only added docs for adapters we maintain. We could potentially link to external docs though

[Rich-Harris]

For comparison, the adapter-auto docs link out to the svelte-adapter-azure-swa docs rather than including them on kit.svelte.dev — I think that's the right approach

[fwang]

Ah makes sense! Removed.

[Rich-Harris]

Thank you! Can you clarify whether this enables true zero-config support? I'm getting conflicting messages from https://docs.sst.dev/start/svelte and https://docs.sst.dev/constructs/SvelteKitSite.

The goal of adapter-auto is to enable people to deploy their SvelteKit apps to different platforms without any setup, but if setup is required (e.g. prefixing vite dev with sst bind) then it's probably better just to include the adapter setup at the same time, rather than adding a new platform to adapter-auto that doesn't actually enable zero-config. We definitely do want to support platforms that are truly zero-config though.

[fwang]

@benmccann @Rich-Harris Appreciate the quick review!

I should clarify that sst bind is an optional step. It's only when you want to interact with other AWS services.

Our goal is to deploy your Svelte app to AWS with zero config. Here's how it works:

cd my-svelte-app
npm create sst
npx sst deploy

The 2nd command npm create sst currently blindly overwrites the svelte.config.js file with a minimal config using the svelte-kit-sst adapter. Given that most users are already using the auto adapter, it'd be much cleaner/safer if SST didn't have to touch the config.

Let me know if that aligns with auto adapter's vision.

[Rich-Harris]

makes sense — thank you!