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/22_clippy/README.md

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+# Clippy
+
+The Clippy tool is a collection of lints to analyze your code so you can catch common mistakes and improve your Rust code.
+
+If you used the installation script for Rustlings, Clippy should be already installed.
+If not you can install it manually via `rustup component add clippy`.
+
+## Further information
+
+- [GitHub Repository](https://github.com/rust-lang/rust-clippy).

exercises/22_clippy/clippy1.rs

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+// The Clippy tool is a collection of lints to analyze your code so you can
+// catch common mistakes and improve your Rust code.
+//
+// For these exercises, the code will fail to compile when there are Clippy
+// warnings. Check Clippy's suggestions from the output to solve the exercise.
+
+fn main() {
+    // TODO: Fix the Clippy lint in this line.
+    let pi: f32 = std::f32::consts::PI;
+    let radius: f32 = 5.0;
+
+    let area = pi * radius.powi(2);
+
+    println!("The area of a circle with radius {radius:.2} is {area:.5}");
+}

exercises/22_clippy/clippy2.rs

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+fn main() {
+    let mut res = 42;
+    let option = Some(12);
+    // TODO: Fix the Clippy lint.
+    if let Some(x) = option {
+        res += x;
+    }
+
+    println!("{res}");
+}

exercises/22_clippy/clippy3.rs

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+// Here are some more easy Clippy fixes so you can see its utility 📎
+// TODO: Fix all the Clippy lints.
+
+#[rustfmt::skip]
+#[allow(unused_variables, unused_assignments)]
+fn main() {
+    let my_option: Option<&str> = None;
+    // Assume that you don't know the value of `my_option`.
+    // In the case of `Some`, we want to print its value.
+    if let Some(val) = my_option {
+        println!("{}", val);
+    }
+
+    let my_arr = &[
+        -1, -2, -3,
+        -4, -5, -6
+    ];
+    println!("My array! Here it is: {my_arr:?}");
+
+    let my_empty_vec: Vec<i32> = vec![];
+    println!("This Vec is empty, see? {my_empty_vec:?}");
+
+    let mut value_a = 45;
+    let mut value_b = 66;
+    // Let's swap these two!
+    std::mem::swap(&mut value_a, &mut value_b);
+    println!("value a: {value_a}; value b: {value_b}");
+}