nu-cli/completions: added completion for $nu (#5303)

crates/nu-engine/src/lib.rs

@@ -11,6 +11,6 @@ pub use documentation::get_full_help;
 pub use env::*;
 pub use eval::{
     eval_block, eval_call, eval_expression, eval_expression_with_input, eval_operator,
-    eval_subexpression,
+    eval_subexpression, eval_variable,
 };
 pub use glob_from::glob_from;