Fixes colinhacks#5544

Adds a mutable z.ext namespace to allow community plugins to extend
Zod without modifying the frozen core z object. This addresses the
issue where direct property assignment fails in production builds.

Problem:
In issue colinhacks#5544, users reported that direct property assignment to
the z namespace (e.g., z.iso.timezone = ...) works in development
but throws "TypeError: Cannot assign to read only property" in
production builds due to object freezing.

Solution:
Implemented z.ext as a dedicated, mutable namespace specifically for
plugins. This approach:

- Is bundler-safe and works in all production environments
- Provides a standardized format for community plugins
- Maintains discoverability through z.ext.* with IDE autocomplete
- Avoids any breaking changes to the core z object

Implementation:
- Created packages/zod/src/v4/classic/plugins.ts with ext namespace
- Exported ext from external.ts to make it available on z
- Added comprehensive tests in plugins.test.ts demonstrating usage
- Included PLUGIN_API.md documentation with examples and best practices

The implementation follows the maintainer's suggestion to use a
dedicated extension namespace rather than a full plugin registration
system, avoiding the issues that have affected libraries like dayjs.

Usage example:
```typescript
// Plugin author
z.ext.cron = () => z.string().refine(...)

// Consumer
import 'my-zod-cron-plugin'
const schema = z.ext.cron()
```