feat: Add preview.package feature flag

Introduces the `nextflow.preview.package` feature flag to enable
the unified package management system. This allows users to opt-in
to the new `package` directive while maintaining backward compatibility
with existing `conda` and `pixi` directives.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>
Signed-off-by: Edmund Miller <[email protected]>

Author: edmundmiller

modules/nf-lang/src/main/java/nextflow/script/dsl/FeatureFlagDsl.java

@@ -55,4 +55,10 @@ Defines the DSL version (`1` or `2`).
     """)
     public boolean previewRecursion;
 
+    @FeatureFlag("nextflow.preview.package")
+    @Description("""
+        When `true`, enables the unified package management system with the `package` directive.
+    """)
+    public boolean previewPackage;
+
 }