Add default introductory code example (exercism#342)

Instead of having logic for a fallback in the backend, we're choosing a
default for the snippet file.

For tracks that have core exercises, we pick the first core exercise.
For tracks without these, we pick the first exercise listed in the config.

Author: Katrina Owen

docs/SNIPPET.txt

@@ -0,0 +1,7 @@
+class HelloWorld {
+  hello() {
+    return 'Hello, World!';
+  }
+}
+
+export default HelloWorld;