fix: add comments prefix if no nested path

The previous fix failed to consider links like
"redd.it/r/.../comments/...".  Those don't really exist in the wild, and
they don't work (when redirects are turned off).  Still, play it safe
and don't add "/comments" prefix unless the path has height 1.

Now, redirects should work for

- redd.it/foo
- redd.it/comments/foo
- redd.it/r/bar/comments/foo

even though the only kind of native link that works is

- redd.it/foo

src/pages/background/background.js

@@ -485,7 +485,7 @@ function redirectReddit(url, initiator, type) {
   } else if (url.host === "redd.it") {
     if (
       redditInstance.includes("teddit") &&
-      !url.pathname.startsWith("/comments/")
+      !url.pathname.match(/^\/\S+\//)
     ) {
       // As of 2021-04-22, redirects for teddit redd.it links don't work unless
       // the path starts with "/comments".  It appears that all links that