present: fix newline parsing for go present

Pressing enter on editable go present code adds a new 'pre' html tag
which isn't being handled properly. This fix in play.js adds a newline
to the html is being parsed whenever it sees a 'pre' tag.

Fixes golang/go#41139

Change-Id: I042d13999f4a8518fb282c4c5bfb3a5197215449
Reviewed-on: https://go-review.googlesource.com/c/tools/+/251697
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

godoc/static/play.js

@@ -12,7 +12,7 @@ function initPlayground(transport) {
       if (n.nodeType === 1) {
         if (n.tagName === 'BUTTON') continue;
         if (n.tagName === 'SPAN' && n.className === 'number') continue;
-        if (n.tagName === 'DIV' || n.tagName == 'BR') {
+        if (n.tagName === 'DIV' || n.tagName === 'BR' || n.tagName === 'PRE') {
           s += '\n';
         }
         s += text(n);