Fixes #20511

When a Svelte file exports snippets through its <script module> block, Vite's dependency scanner fails with an esbuild error:

✘ [ERROR] "foo" is not declared in this file
 script:/path/to/Component.svelte?id=0:2:12:
 2 │ export { foo };
 ╵ ~~~

This happens because snippets are defined in the template using {#snippet foo()} syntax, but the scanner only sees the script block where export { foo } references an undeclared identifier.

Solution

This PR adds special handling for Svelte files containing #snippet:

1. Detection: When scanning Svelte files with <script module> blocks, check if the file content contains #snippet
2. Virtual Module Export: For files with snippets, add export * from 'svelte-snippet-dummy:filename' to handle snippet exports that aren't defined in the script block
3. Externalization: Add an esbuild onResolve handler to externalize any modules starting with svelte-snippet-dummy

Example

Before this fix, this Svelte file would cause scanning to fail:

<script module>
 export { mySnippet };
</script>
{#snippet mySnippet()}
 <p>Hello from snippet!</p>
{/snippet}

After the fix, the scanner generates:

export * from 'svelte-snippet-dummy:/path/to/Component.svelte'
import "virtual:Component.svelte?id=0"
export default {}

The virtual module is externalized, preventing esbuild from trying to resolve snippet exports that don't exist in the script block.

Testing

• Added integration test with actual Svelte file containing snippets
• Test verifies that scanning completes without errors for Svelte files with #snippet syntax
• All existing tests continue to pass
• Manual verification confirms the fix resolves the scanning error

This change is backward compatible and only affects Svelte files that contain #snippet syntax.

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