Fixed paths in generated main.cjs for the default sandbox on Windows

[Dschungelabenteuer]

See relevant discussion on Discord

When generating the default sandbox on a Windows environment, it creates Windows paths in the output main.cjs. Those paths are not being resolved into absolute paths from Vite, preventing the example Storybook from starting.

{
    directory: "../template-stories\\addons\\actions",
    titlePrefix: "addons\\actions",
    files: "**/*.@(mdx|stories.@(js|jsx|ts|tsx))"
}
previewAnnotations: [
  ".\\src\\stories\\components",
  "./template-stories\\lib\\store\\preview.ts",
  "./template-stories\\addons\\toolbars\\preview.ts"
],

What I did

I simply wrapped several paths with slash so that it actually uses POSIX paths

How to test

1. Run the default sandbox yarn install on Windows
2. See if it works out of the box!

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]