feat: Add support for loop statements.

src/main.rs

@@ -5,7 +5,7 @@ pub mod parser;
 pub mod token;
 
 fn main() {
-    let mut parser = Parser::new("if a < 12 { let b = 10; } else { let c = 20; }");
+    let mut parser = Parser::new("loop {}");
 
     println!("{:#?}", parser.parse_statement());
 }