Add solutions to intro and variables

solutions/00_intro/intro1.rs

@@ -1 +1,2 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+// The exercise `intro1` only requires entering `n` in the terminal to go to the next exercise.
+// It is just an introduction to how Rustlings works.

solutions/00_intro/intro2.rs

@@ -1 +1,4 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+fn main() {
+    // `println!` instead of `printline!`.
+    println!("Hello world!");
+}