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",
@@ -427,11 +427,14 @@ pub fn evaluate_repl(
 
         entry_num += 1;
 
-        if entry_num == 1 && show_banner {
-            println!(
-                "Startup Time: {}",
-                format_duration(entire_start_time.elapsed().as_nanos() as i64)
-            );
+        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(engine_state.get_startup_time())
+                );
+            }
         }
 
         start_time = std::time::Instant::now();