fix indentation in pre in preview

Again, this is a dodgy hack: replace spaces with dot with the color
exactly like the background, because otherwise ST ignores it, even in
pre

resources/stylesheet.css

@@ -38,3 +38,6 @@ pre code {
     padding: 0;
 }
 
+i.space {
+    color: var(--very-light-bg);
+}