Update SHLVL (only when interactive) on startup (#14404)

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# Description
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Make NuShell correctly inherit and update `SHLVL` from other shells
(obviously including itself) in Unix environment.

See issue #14384

# User-Facing Changes
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helps us keep track of breaking changes. -->

None

# Tests + Formatting
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New code formatted.

New feature works well in interactive usage.

# After Submitting
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src/main.rs

@@ -280,6 +280,16 @@ fn main() -> Result<()> {
         Value::string(env!("CARGO_PKG_VERSION"), Span::unknown()),
     );
 
+    // Add SHLVL if interactive
+    if engine_state.is_interactive {
+        let mut shlvl = engine_state
+            .get_env_var("SHLVL")
+            .map(|x| x.as_str().unwrap_or("0").parse::<i64>().unwrap_or(0))
+            .unwrap_or(0);
+        shlvl += 1;
+        engine_state.add_env_var("SHLVL".to_string(), Value::int(shlvl, Span::unknown()));
+    }
+
     if parsed_nu_cli_args.no_std_lib.is_none() {
         load_standard_library(&mut engine_state)?;
     }