ci: publish preview packages via `pkg.pr.new`

[aryaemami59]

Summary

CodeSandbox CI is deprecated and fully shuts down on July 15, 2026. We rely on it to publish installable preview builds of our packages for every commit/PR. Adopt pkg.pr.new, a GitHub Actions-native equivalent, and retire CodeSandbox CI before the
shutdown.

• Add a pkg.pr.new workflow that publishes preview builds per commit/PR, installable via npm install https://pkg.pr.new/@reduxjs/toolkit@{PR}.
• Remove .codesandbox/ci.json and any remaining CodeSandbox CI references.