Add the ability to remove and list aliases (#3879)

* Add the ability to remove and list aliases

* Fix failing unit tests

* Add a test to check unalias shadowing blocks

crates/nu-completion/src/engine.rs

@@ -317,6 +317,10 @@ mod tests {
             todo!()
         }
 
+        fn remove_alias(&self, _name: &str) {
+            todo!()
+        }
+
         fn add_definition(&self, _block: Arc<Block>) {}
 
         fn get_definitions(&self) -> Vec<Arc<Block>> {