Add Rust section and rustlings to interactive tutorials (#4022)

* Add rustlings to interactive tutorials

Add a new section for rust, and add the rustlings tutorial

* Fix spacing between header and content

free-programming-interactive-tutorials-en.md

@@ -28,6 +28,7 @@
 * [PHP](#php)
 * [Python](#python)
 * [Ruby](#ruby)
+* [Rust](#rust)
 * [Scala](#scala)
 * [SQL](#sql)
 * [Vim](#vim)
@@ -221,6 +222,11 @@
 * [Try Ruby](http://tryruby.org)
 
 
+### Rust
+
+* [Rustlings](https://github.com/rust-lang/rustlings)
+
+
 ### Scala
 
 * [A Tour of Scala - an interactive scala tutorial](https://scalatutorials.com/tour/)