[WIP] Add type safety for builder configurations with discriminated unions

[Copilot]

Thanks for the feedback on #80. I've created this new PR, which merges into #80, to address your comment. I will work on the changes and keep this PR's description up to date as I make progress.

Original PR: #80
Triggering comment (#80 (comment)):

@copilot Can you try to fix the build error?

@workflow/sveltekit:build: src/builder.ts(25,7): error TS2322: Type '"sveltekit"' is not assignable to type '"standalone" | "vercel-build-output-api" | "next"'.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[changeset-bot]

⚠️ No Changeset found

Latest commit: a861df0

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.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Comments	Updated (UTC)
example-nextjs-workflow-turbopack	Error			Nov 4, 2025 7:08am
example-nextjs-workflow-webpack	Error			Nov 4, 2025 7:08am
example-workflow	Error			Nov 4, 2025 7:08am
workbench-nitro-workflow	Error			Nov 4, 2025 7:08am
workbench-nuxt-workflow	Error			Nov 4, 2025 7:08am
workbench-sveltekit-workflow	Error			Nov 4, 2025 7:08am
workbench-vite-workflow	Error			Nov 4, 2025 7:08am