fix(macros): Mark the main generate function as must_use

The generated FileTree should be used, calling the function without
using it is useless.

src/macros/mod.rs

@@ -61,6 +61,7 @@ impl TrixyConfig {
     ///                         /* other config settings */;
     /// trixy_config.generate();
     /// ```
+    #[must_use]
     pub fn generate(&self) -> FileTree {
         let mut file_tree = FileTree::new();