feat: Add support for expression and let statements.

src/main.rs

@@ -5,7 +5,7 @@ pub mod parser;
 pub mod token;
 
 fn main() {
-    let mut parser = Parser::new("user.age >= 18");
+    let mut parser = Parser::new("let index = 12 + 3;");
 
-    println!("{:#?}", parser.parse_expression(0));
+    println!("{:#?}", parser.parse_statement());
 }