feat: Add support for function declarations.

src/main.rs

@@ -5,7 +5,7 @@ pub mod parser;
 pub mod token;
 
 fn main() {
-    let mut parser = Parser::new("{ let return_code: i32 = 12; return return_code; }");
+    let mut parser = Parser::new(include_str!("../example/simple.bky"));
 
-    println!("{:#?}", parser.parse_statement());
+    println!("{:#?}", parser.parse_declaration());
 }