crosh_builtin: declare an options page

The options page has been around for 5+ years, but only accessible if
you typed the URL directly, or used the Ctrl-Shift-P shortcut added in
the last year or so.

While we've been updating the manifest files in the nassh repo, we had
forgotten about this one.  Although it wasn't useful before the recent
Terminal/Crostini work as crosh hasn't been a visible "app", only via
the Ctrl-Alt-T shortcut on a Chromebook.

Bug: 843613
Change-Id: I32a67cd85d5eafe147672cd93c35dea0cc5edcdd
Reviewed-on: https://chromium-review.googlesource.com/1061618
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559414}

chrome/browser/resources/chromeos/crosh_builtin/manifest.json

@@ -10,6 +10,7 @@
   "default_locale": "en",
   "description": "Built-in terminal for crosh.",
   "offline_enabled": true,
+  "options_page": "html/nassh_preferences_editor.html",
   "incognito": "split",
   "permissions": [
     "clipboardRead",