SvelteKit: Fix HMR not working

[JReinhold]

Closes #25029

What I did

Changed the Vite plugin that adds aliases mocks from returning a full, merged config to only returning a partial config. I don't know why this fixes HMR. 🤷
Also removed enforce: 'post' on it, not because it contributed to the issue, but I didn't see any noticeable change with or without it, it didn't seem to do anything.

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This pull request has been released as version 0.0.0-pr-25031-sha-f8489ef6. Install it by pinning all your Storybook dependencies to that version.

More information

Published version	0.0.0-pr-25031-sha-f8489ef6
Triggered by	@JReinhold
Repository	storybookjs/storybook
Branch	25029-bug-sveltekit-hot-reload-not-working-after-upgrading-to-storybook-76
Commit	f8489ef6
Datetime	Wed Nov 29 09:43:28 UTC 2023 (1701251008)
Workflow run	7030704506

To request a new release of this pull request, mention the @storybookjs/core team.

core team members can create a new canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=25031