Initialize Rust implementation of Lox interpreter

This initial commit sets up the basic structure for a Rust
implementation of the Lox interpreter, including:

- Lexer for tokenizing source code - Parser for building AST - Basic
interpreter functionality - Environment for variable storage

The core components include source tracking, error handling, and basic
expression evaluation.

src/backend/environment.rs

@@ -0,0 +1,44 @@
+use std::collections::HashMap;
+
+use crate::{
+    frontend::{source_registry::SourceSlice, tokens::LiteralValue},
+    result::{LoxError, LoxResult},
+};
+
+pub struct Environment<'a> {
+    values: HashMap<String, LiteralValue>,
+    parent: Option<&'a Environment<'a>>,
+}
+
+impl<'a> Environment<'a> {
+    pub fn new() -> Self {
+        Environment {
+            values: HashMap::new(),
+            parent: None,
+        }
+    }
+
+    pub fn new_with_parent(parent: &'a Environment<'a>) -> Self {
+        Environment {
+            values: HashMap::new(),
+            parent: Some(parent),
+        }
+    }
+
+    pub fn get(&self, name: &str) -> LoxResult<LiteralValue> {
+        match self.values.get(name) {
+            Some(value) => Ok(value.clone()),
+            None => match self.parent {
+                Some(parent) => parent.get(name),
+                None => Err(LoxError::RuntimeError {
+                    source_slice: SourceSlice::default(), // todo change this to the actual source slice
+                    message: format!("Undefined variable '{}'", name),
+                }),
+            },
+        }
+    }
+
+    pub fn set(&mut self, name: String, value: LiteralValue) {
+        self.values.insert(name, value);
+    }
+}