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-protocol/src/engine/engine_state.rs

@@ -6,6 +6,7 @@ use crate::{
         CachedFile, Command, CommandType, EnvVars, OverlayFrame, ScopeFrame, Stack, StateDelta,
         Variable, Visibility, DEFAULT_OVERLAY_NAME,
     },
+    eval_const::create_nu_constant,
     BlockId, Category, Config, DeclId, Example, FileId, HistoryConfig, Module, ModuleId, OverlayId,
     ShellError, Signature, Span, Type, Value, VarId, VirtualPathId,
 };
@@ -753,8 +754,8 @@ impl EngineState {
         var.const_val.as_ref()
     }
 
-    pub fn set_variable_const_val(&mut self, var_id: VarId, val: Value) {
-        self.vars[var_id].const_val = Some(val);
+    pub fn generate_nu_constant(&mut self) {
+        self.vars[NU_VARIABLE_ID].const_val = Some(create_nu_constant(self, Span::unknown()));
     }
 
     pub fn get_decl(&self, decl_id: DeclId) -> &dyn Command {

crates/nu-protocol/src/errors/cli_error.rs

@@ -41,7 +41,6 @@ pub fn report_error_new(
     error: &(dyn miette::Diagnostic + Send + Sync + 'static),
 ) {
     let working_set = StateWorkingSet::new(engine_state);
-
     report_error(&working_set, error);
 }
 

crates/nu-protocol/src/eval_const.rs

@@ -12,7 +12,7 @@ use std::{
 };
 
 /// Create a Value for `$nu`.
-pub fn create_nu_constant(engine_state: &EngineState, span: Span) -> Result<Value, ShellError> {
+pub(crate) fn create_nu_constant(engine_state: &EngineState, span: Span) -> Value {
     fn canonicalize_path(engine_state: &EngineState, path: &Path) -> PathBuf {
         #[allow(deprecated)]
         let cwd = engine_state.current_work_dir();
@@ -200,7 +200,7 @@ pub fn create_nu_constant(engine_state: &EngineState, span: Span) -> Result<Valu
         },
     );
 
-    Ok(Value::record(record, span))
+    Value::record(record, span)
 }
 
 fn eval_const_call(