Show code blocks at full height

...well, for any reasonable height anyway.

When scrollbars appear the refreshing output tends to get janky, so
just allow the code region (called a fence in markdown, apparently)
to be up to 9999 lines tall.

src/chap/commands/tui.css

@@ -36,6 +36,9 @@ Markdown:focus-within {
 Markdown {
     border-left: heavy transparent;
 }
+MarkdownFence {
+    max-height: 9999;
+}
 Footer {
     dock: top;
 }