Sandboxes: Replace yarn with npm

[JReinhold]

WIP

What I did

This PR removes any Yarn specific stuff from generated sandboxes, to then instead use the default npm. There are multiple reasons for this:

1. Sandboxes are generated with Yarn v1, which is unmaintained and has multiple issues. (I experienced this today myself when reproducing Theming: Add theme variable to set the preview background color #24575
2. StackBlitz only support Yarn v1, so even if we upgraded, StackBlitz wouldn't be affected.
3. Yarn v1 has no real advantage over npm today. However npm is more widely used and will have a smoother learning curve for new users.

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 PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

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

[JReinhold]

This requires more effort, especially to get sandbox generation working in the monorepo. Closing for now.