feat: add environment variable support for allowedOrigins configs
- Add NEXT_ALLOWED_DEV_ORIGINS env var to inject allowedDevOrigins
- Add NEXT_SERVER_ACTIONS_ALLOWED_ORIGINS env var to inject experimental.serverActions.allowedOrigins
- Both env vars accept comma-separated lists of origins
- Values from env vars are merged with config values
- Add comprehensive unit tests for the new functionality
Co-authored-by: Jimmy Lai <hello@jimmyl.ai>