Refactor the CLI code a bit (#12782)

# Description
Refactors the code in `nu-cli`, `main.rs`, `run.rs`, and few others.
Namely, I added `EngineState::generate_nu_constant` function to
eliminate some duplicate code. Otherwise, I changed a bunch of areas to
return errors instead of calling `std::process::exit`.

# User-Facing Changes
Should be none.

crates/nu-engine/src/env.rs

@@ -32,7 +32,7 @@ enum ConversionResult {
 /// It returns Option instead of Result since we do want to translate all the values we can and
 /// skip errors. This function is called in the main() so we want to keep running, we cannot just
 /// exit.
-pub fn convert_env_values(engine_state: &mut EngineState, stack: &Stack) -> Option<ShellError> {
+pub fn convert_env_values(engine_state: &mut EngineState, stack: &Stack) -> Result<(), ShellError> {
     let mut error = None;
 
     let mut new_scope = HashMap::new();
@@ -85,7 +85,11 @@ pub fn convert_env_values(engine_state: &mut EngineState, stack: &Stack) -> Opti
         });
     }
 
-    error
+    if let Some(err) = error {
+        Err(err)
+    } else {
+        Ok(())
+    }
 }
 
 /// Translate one environment variable from Value to String