FEATURE: add the startup time to $nu (#8353)

# Description
in https://github.com/nushell/nushell/issues/8311 and the discord
server, the idea of moving the default banner from the `rust` source to
the `nushell` standar library has emerged 😋

however, in order to do this, one need to have access to all the
variables used in the default banner => all of them are accessible
because known constants, except for the startup time of the shell, which
is not anywhere in the shell...

#### this PR adds exactly this, i.e. the new `startup_time` to the `$nu`
variable, which is computed to have the exact same value as the value
shown in the banner.

## the changes
in order to achieve this, i had to
- add `startup_time` as an `i64` to the `EngineState` => this is, to the
best of my knowledge, the easiest way to pass such an information around
down to where the banner startup time is computed and where the `$nu`
variable is evaluated
- add `startup-time` to the `$nu` variable and use the `EngineState`
getter for `startup_time` to show it as a `Value::Duration`
- pass `engine_state` as a `&mut`able argument from `main.rs` down to
`repl.rs` to allow the setter to change the value of `startup_time` =>
without this, the value would not change and would show `-1ns` as the
default value...
- the value of the startup time is computed in `evaluate_repl` in
`repl.rs`, only once at the beginning, and the same value is used in the
default banner 👌

# User-Facing Changes
one can now access to the same time as shown in the default banner with
```bash
$nu.startup-time
```

# Tests + Formatting
- 🟢 `cargo fmt --all`
- 🟢 `cargo clippy --workspace -- -D warnings -D
clippy::unwrap_used -A clippy::needless_collect`
- 🟢 `cargo test --workspace`

# After Submitting
```
$nothing
```

crates/nu-cli/src/repl.rs

@@ -414,7 +414,7 @@ pub fn evaluate_repl(
         );
 
         start_time = std::time::Instant::now();
-        let config = engine_state.get_config();
+        let config = &engine_state.get_config().clone();
         let prompt = prompt_update::update_prompt(config, engine_state, stack, &mut nu_prompt);
         perf(
             "update_prompt",
@@ -427,12 +427,15 @@ pub fn evaluate_repl(
 
         entry_num += 1;
 
-        if entry_num == 1 && show_banner {
+        if entry_num == 1 {
+            engine_state.set_startup_time(entire_start_time.elapsed().as_nanos() as i64);
+            if show_banner {
                 println!(
                     "Startup Time: {}",
-                format_duration(entire_start_time.elapsed().as_nanos() as i64)
+                    format_duration(engine_state.get_startup_time())
                 );
             }
+        }
 
         start_time = std::time::Instant::now();
         let input = line_editor.read_line(prompt);

crates/nu-cli/tests/completions.rs

@@ -524,7 +524,7 @@ fn variables_completions() {
     // Test completions for $nu
     let suggestions = completer.complete("$nu.", 4);
 
-    assert_eq!(11, suggestions.len());
+    assert_eq!(12, suggestions.len());
 
     let expected: Vec<String> = vec![
         "config-path".into(),
@@ -537,6 +537,7 @@ fn variables_completions() {
         "os-info".into(),
         "pid".into(),
         "scope".into(),
+        "startup-time".into(),
         "temp-path".into(),
     ];
 

crates/nu-engine/src/nu_variable.rs

@@ -36,6 +36,7 @@ impl LazyRecord for NuVariable {
         cols.push("temp-path");
         cols.push("pid");
         cols.push("os-info");
+        cols.push("startup-time");
 
         cols.push("is-interactive");
         cols.push("is-login");
@@ -188,6 +189,10 @@ impl LazyRecord for NuVariable {
                 val: self.engine_state.is_login,
                 span: self.span,
             }),
+            "startup-time" => Ok(Value::Duration {
+                val: self.engine_state.get_startup_time(),
+                span: self.span(),
+            }),
             _ => err(&format!("Could not find column '{column}'")),
         }
     }

crates/nu-protocol/src/engine/engine_state.rs

@@ -147,6 +147,7 @@ pub struct EngineState {
     pub regex_cache: Arc<Mutex<LruCache<String, Regex>>>,
     pub is_interactive: bool,
     pub is_login: bool,
+    startup_time: i64,
 }
 
 // The max number of compiled regexes to keep around in a LRU cache, arbitrarily chosen
@@ -199,6 +200,7 @@ impl EngineState {
             ))),
             is_interactive: false,
             is_login: false,
+            startup_time: -1,
         }
     }
 
@@ -1015,6 +1017,14 @@ impl EngineState {
     pub fn get_file_contents(&self) -> &Vec<(Vec<u8>, usize, usize)> {
         &self.file_contents
     }
+
+    pub fn get_startup_time(&self) -> i64 {
+        self.startup_time
+    }
+
+    pub fn set_startup_time(&mut self, startup_time: i64) {
+        self.startup_time = startup_time;
+    }
 }
 
 /// A temporary extension to the global state. This handles bridging between the global state and the

src/main.rs

@@ -245,6 +245,6 @@ fn main() -> Result<()> {
         )
     } else {
         engine_state.is_interactive = true;
-        run_repl(engine_state, parsed_nu_cli_args, entire_start_time)
+        run_repl(&mut engine_state, parsed_nu_cli_args, entire_start_time)
     }
 }

src/run.rs

@@ -194,7 +194,7 @@ pub(crate) fn run_file(
 }
 
 pub(crate) fn run_repl(
-    mut engine_state: nu_protocol::engine::EngineState,
+    engine_state: &mut nu_protocol::engine::EngineState,
     parsed_nu_cli_args: command::NushellCliArgs,
     entire_start_time: std::time::Instant,
 ) -> Result<(), miette::ErrReport> {
@@ -203,7 +203,7 @@ pub(crate) fn run_repl(
 
     if parsed_nu_cli_args.no_config_file.is_none() {
         setup_config(
-            &mut engine_state,
+            engine_state,
             &mut stack,
             #[cfg(feature = "plugin")]
             parsed_nu_cli_args.plugin_file,
@@ -226,7 +226,7 @@ pub(crate) fn run_repl(
 
     let start_time = std::time::Instant::now();
     let ret_val = evaluate_repl(
-        &mut engine_state,
+        engine_state,
         &mut stack,
         config_files::NUSHELL_FOLDER,
         parsed_nu_cli_args.execute,