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/01_variables/README.md

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+# Variables
+
+In Rust, variables are immutable by default.
+When a variable is immutable, once a value is bound to a name, you can't change that value.
+You can make them mutable by adding `mut` in front of the variable name.
+
+## Further information
+
+- [Variables and Mutability](https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html)

exercises/01_variables/variables1.rs

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+fn main() {
+    // TODO: Add the missing keyword.
+    let x = 5;
+
+    println!("x has the value {x}");
+}

exercises/01_variables/variables2.rs

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+fn main() {
+    // TODO: Change the line below to fix the compiler error.
+    let x = 10;
+
+    if x == 10 {
+        println!("x is ten!");
+    } else {
+        println!("x is not ten!");
+    }
+}

exercises/01_variables/variables3.rs

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+fn main() {
+    // TODO: Change the line below to fix the compiler error.
+    let x: i32 = 0;
+
+    println!("Number {x}");
+}

exercises/01_variables/variables4.rs

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+// TODO: Fix the compiler error.
+fn main() {
+    let mut x = 3;
+    println!("Number {x}");
+
+    x = 5; // Don't change this line
+    println!("Number {x}");
+}

exercises/01_variables/variables5.rs

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+fn main() {
+    let number = "T-H-R-E-E"; // Don't change this line
+    println!("Spell a number: {number}");
+
+    // TODO: Fix the compiler error by changing the line below without renaming the variable.
+    let number = 3;
+    println!("Number plus two is: {}", number + 2);
+}

exercises/01_variables/variables6.rs

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+// TODO: Change the line below to fix the compiler error.
+const NUMBER: i32 = 3;
+
+fn main() {
+    println!("Number: {NUMBER}");
+}