Use h-scrolling instead of wrap in code block

If a line in a code block exceeds the widdth, hide overflow and add
horizontal scrollbars instead of wrapping the line.

Signed-off-by: Anton Strömkvist <anton@stromkvist.com>

css/prosemirror.scss

@@ -171,7 +171,8 @@ div.ProseMirror {
 	}
 
 	pre {
-		white-space: pre-wrap;
+		white-space: pre;
+		overflow-x: auto;
 		background-color: var(--color-background-dark);
 		border-radius: var(--border-radius);
 		padding: 1em 1.3em;