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-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