Add export alias and export extern (#4878)

* export alias

* export extern

crates/nu-protocol/src/exportable.rs

@@ -1,6 +1,7 @@
-use crate::{BlockId, DeclId};
+use crate::{AliasId, BlockId, DeclId};
 
 pub enum Exportable {
     Decl(DeclId),
+    Alias(AliasId),
     EnvVar(BlockId),
 }