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/middleend/mod.rs

@@ -0,0 +1 @@
+pub mod crawler;