Set up GitHub workflow for npm deployment

[TommyBez]

A GitHub workflow .github/workflows/deploy.yml was created to automate plugin deployment to npm.

• Triggers: The workflow runs on pushes to the main branch affecting packages/plugin/ and can be manually dispatched via workflow_dispatch.
• Deployment Logic:
  • It sets up Node.js and pnpm, caches dependencies, and builds the plugin using pnpm build --filter=stagewise-animation-plugin.
  • A version check compares the current packages/plugin/package.json version against the version published on npm.
  • Publishing to npm and creating a GitHub release only occurs if the version has changed, ensuring deployments happen only on version bumps.
• The "private": true field was removed from packages/plugin/package.json to enable npm publishing.
• A deployment-setup.md guide was added, detailing how to configure the required NPM_TOKEN GitHub secret and explaining the workflow's operation and features.