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-command/src/commands/core_commands/nu_signature.rs

@@ -14,6 +14,7 @@ impl WholeStreamCommand for Command {
             .switch("stdin", "redirect stdin", None)
             .switch("skip-plugins", "do not load plugins", None)
             .switch("no-history", "don't save history", None)
+            .switch("perf", "show startup performance metrics", None)
             .named(
                 "commands",
                 SyntaxShape::String,