Syntax highlighting (#691)

* Syntax highlighting mostly... Exists.

* Add dependency to dockerfile

* Handle non-existent languages better

* Make the default a bit nicer

* Improve highlighting. Clean up function

* Add dark theme, add the comment scope to the allowed classes

* update build env

* Address review comments

* Use find_syntax_by_token which produces the desired behavior

* Change flat_map into flatten
(commit cargo.lock)

assets/themes/default/_dark_variables.scss

@@ -17,3 +17,12 @@ $text-color: $white;
 $primary: $purple;
 $primary-text-color: $white; // text color on primary background (buttons for instance)
 $success-color: $blue;
+
+//Code Highlighting
+
+$code-keyword-color: #f79ac1;
+$code-source-color: #a6f0ab;
+$code-constant-color: #dfec84;
+$code-operator-color: #eddf95;
+$code-string-color: #f2ae60;
+$code-comment-color: #a3b4f9;