v6.4.0-alpha1: chore: update npm publishing to use OIDC authentication (#501)

This updates the CI workflow to use OIDC authentication for npm publishing instead of static tokens. This is more secure and follows GitHub's recommended practices.

Changes:
- Added 'permissions: id-token: write' to publish job
- Removed NPM_TOKEN environment variable from publish job
- Removed 'npm config set' command that configured static token authentication
- Updated npm publish commands to use 'npx -y npm@latest publish' wrapped in a publish() function
- Preserved all existing jobs, steps, conditions, and INTERCOM_API_KEY environment variable