Test command names and search terms for redundancy (#6380)

* Test commands for proper names and search terms

Assert that the `Command.name()` is equal to `Signature.name`

Check that search terms are not just substrings of the command name as
they would not help finding the command.

* Clean up search terms

Remove redundant terms that just replicate the command name.
Try to eliminate substring between search terms, clean up where
necessary.

crates/nu-command/src/math/avg.rs

@@ -21,7 +21,7 @@ impl Command for SubCommand {
     }
 
     fn search_terms(&self) -> Vec<&str> {
-        vec!["average", "mean"]
+        vec!["average", "mean", "statistics"]
     }
 
     fn run(