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-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}'")),
         }
     }