[fix] <a> tag in footer to open in a new tab (sveltejs#155)

If the content of the sidebar is long enough the users might misclick
and choose the *Edit this page* link instead of the *Next: Lesson* link.
By doing so, the link will open within the user lesson tab and even if
they go back to their previous tab the progress will be lost.

This patch makes the <a> tag on the footer open in a new tab, so if
the user misclicks on the link they can close it and continue normally.

Author: mateusabelli

src/routes/tutorial/[slug]/Sidebar.svelte

@@ -73,7 +73,12 @@
 </div>
 
 <footer>
-	<a class="edit" href="https://github.com/sveltejs/learn.svelte.dev/tree/main/{exercise.dir}">
+	<a
+		target="_blank"
+		rel="noreferrer"
+		class="edit"
+		href="https://github.com/sveltejs/learn.svelte.dev/tree/main/{exercise.dir}"
+	>
 		Edit this page
 	</a>
 </footer>