feat: throw error if routes are configured in _routes.json

[kettei-sproutty]

Close #12321

I don't know if it was the best choice, but I opted for throw error only if _routes.json exists and there are no configured routes in svelte.config.

Let me know if you think Changelogs should be generated.

I don't think it's related, but pnpm test fails on

Watching src/lib for changes...

✘ • • • •   (14 / 15)

FAIL  "watches for changes"
Expected values to be deeply equal:  (equal)

Actual:
--<script·lang="ts">export·let·answer:·number</script>
Expected:
++<script>export·let·answer;
++</script>
        
@sveltejs/[email protected] test: `uvu test "^index.js$"

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[changeset-bot]

🦋 Changeset detected

Latest commit: 2e99678

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-cloudflare	Minor

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[benmccann]

will adapter-cloudflare-workers need the same change?

don't worry about the test failure. the tests are flaky

yes, we'll need a changeset

[kettei-sproutty]

will adapter-cloudflare-workers need the same change?

don't worry about the test failure. the tests are flaky

yes, we'll need a changeset

The adapter-cloudflare-workers shouldn't require this change because as far as i saw its configuration is managed via wrangler.toml in both svelte and cloudflare docs.

I will generate the changelogs asap than :)
Thank you for the review!

[eltigerchino]

Thank you!