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/_variables.scss

@@ -27,3 +27,11 @@ $margin: 0 $horizontal-margin;
 $route159: "Route159", serif;
 $playfair: "Playfair Display", serif;
 $lora: "Lora", serif;
+
+//Code Highlighting
+$code-keyword-color: #45244a;
+$code-source-color: #4c588c;
+$code-constant-color: scale-color(magenta,$lightness:-5%);
+$code-operator-color: scale-color($code-source-color,$lightness:-5%);
+$code-string-color: #8a571c;
+$code-comment-color: #1c4c8a;