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/frontend/mod.rs

@@ -0,0 +1,5 @@
+pub mod ast;
+pub mod lexer;
+pub mod parser;
+pub mod source_registry;
+pub mod tokens;