add performance metrics for measuring startup time (#3854)

* add performance metrics for measureing startup time

* removed some comments

* update so tests pass

* update default.toml for tests, merged main

* fix clippy lints

* wording changes

crates/nu-test-support/src/playground/director.rs

@@ -67,6 +67,7 @@ impl Director {
                 config_file.to_str().expect("failed to convert."),
             ]);
         }
+        process.arg("--perf");
 
         director.executable = Some(process);
         director