added function and variable lessons.

the_book/variables/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "variables"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

the_book/variables/src/main.rs

@@ -0,0 +1,12 @@
+fn main() {
+    let x = 5;
+
+    let x = x + 1;
+
+    {
+        let x = x * 2;
+        println!("The value of x in the inner scope is: {}", x);
+    }
+
+    println!("The value of x is: {}", x);
+}