feat: add deprecation warning for v4 content collections in v5
Emit console warning when collections are defined without a loader,
signaling that they're using deprecated v4 backwards compatibility mode.
This helps users discover and migrate their legacy collections before
upgrading to v6, where the escape hatch will require explicit opt-in
with legacy.collectionsBackwardsCompat flag.
Warning only shows in dev mode (NODE_ENV !== 'production') to avoid
spam in production builds.