Add .idea to gitignore and prep return value types Add middleend module

for AST return value tracking

The changes introduce support for tracking return values and labels
across AST nodes. The key changes include:

- Adding ReturnValue type to BaseValue - Adding return_type field to
LoxFunction - Adding label field to Block statements - Creating
middleend crawler module for AST traversal Add return value types to AST

This accurately captures both changes: adding `.idea/` to gitignore and
adding return value types to the AST structure. The changes include
adding return value fields to statement nodes and adjusting the
interpreter logic accordingly.

src/common/base_value.rs

@@ -271,6 +271,10 @@ pub enum BaseValue {
     Nil,
     Function(LoxFunction),
     NativeFunction(NativeFunction),
+    Return {
+        value: Box<BaseValue>,
+        labels: Vec<String>,
+    },
 }
 
 impl BaseValue {
@@ -292,6 +296,9 @@ impl Display for BaseValue {
             BaseValue::Nil => write!(f, "nil"),
             BaseValue::Function(..) => write!(f, "<fn lox function>"),
             BaseValue::NativeFunction(..) => write!(f, "<native native function>"),
+            BaseValue::Return { labels, .. } => {
+                write!(f, "<return value [return:{}]>", labels.join(":"))
+            }
         }
     }
 }
@@ -299,6 +306,7 @@ impl Display for BaseValue {
 #[derive(Debug, Clone, PartialEq)]
 pub struct LoxFunction {
     pub parameters: Vec<(String, String)>,
+    pub return_type: Option<String>,
     pub body: AstNode<Stmt>,
     pub closure: Option<EnvironmentStack>,
     pub guard: Option<Box<Expr>>,
@@ -313,6 +321,7 @@ impl LoxFunction {
     ) -> Self {
         LoxFunction {
             parameters,
+            return_type: None,
             body,
             closure,
             guard,
@@ -322,6 +331,7 @@ impl LoxFunction {
     pub fn anonymous_function(parameters: Vec<(String, String)>, body: AstNode<Stmt>) -> Self {
         LoxFunction {
             parameters,
+            return_type: None,
             body,
             closure: None,
             guard: None,