return const values from scope variables (#14577)

Fixes #14542

# User-Facing Changes

Constant values are no longer missing from `scope variables` output
when the IR evaluator is enabled:

```diff
const foo = 1
scope variables | where name == "$foo" | get value.0 | to nuon
-null
+int
```

crates/nu-engine/src/scope.rs

@@ -56,11 +56,12 @@ impl<'e, 's> ScopeData<'e, 's> {
             let var_type = Value::string(var.ty.to_string(), span);
             let is_const = Value::bool(var.const_val.is_some(), span);
 
-            let var_value = if let Ok(val) = self.stack.get_var(**var_id, span) {
-                val
-            } else {
-                Value::nothing(span)
-            };
+            let var_value = self
+                .stack
+                .get_var(**var_id, span)
+                .ok()
+                .or(var.const_val.clone())
+                .unwrap_or(Value::nothing(span));
 
             let var_id_val = Value::int(var_id.get() as i64, span);