Add initial solution files for Rustlings exercises and quizzes

- Created empty solution files for various exercises in strings, modules, hashmaps, options, error handling, generics, traits, lifetimes, tests, iterators, smart pointers, threads, macros, clippy, conversions, and quizzes.
- Each solution file contains a main function with a comment indicating that it will be automatically filled after completing the exercise.
- Added a README.md file to provide information about the solutions and their purpose.

exercises/21_macros/macros4.rs

@@ -0,0 +1,15 @@
+// TODO: Fix the compiler error by adding one or two characters.
+#[rustfmt::skip]
+macro_rules! my_macro {
+    () => {
+        println!("Check out my macro!");
+    };
+    ($val:expr) => {
+        println!("Look at this other macro: {}", $val);
+    }
+}
+
+fn main() {
+    my_macro!();
+    my_macro!(7777);
+}