Scroll long code blocks

This sets the width of code blocks to be the width of the page, and
adds a scrollbar for long blocks. Article `c146e768bb41` has some
examples.

I could have also wrapped the codeblocks, but as pointed out by
[~kaki87] this often reduces readability. Hence: scrollbars.

[~kaki87]: https://todo.sr.ht/~edwardloveall/Scribe/6#event-188395

src/css/app.css

@@ -34,3 +34,10 @@ figure iframe {
 footer p span {
   margin-right: 1em;
 }
+
+pre {
+  background-color: rgba(127, 127, 127, 0.1);
+  padding: 1em;
+  overflow-x: scroll;
+  width: 100%;
+}