Whitelist "vscode://" as an allowed Editor protocol

Summary:
See PHI1647, which asks for "vscode://" to be a configurable protocol on hosted Phacility instances.

I made the configuration editable in D21008, but this can reasonably just come upstream too.

Test Plan: Viewed config in Config, set my editor URI to `vscode://blahblah`.

Differential Revision: https://secure.phabricator.com/D21009

src/applications/config/option/PhabricatorSecurityConfigOptions.php

@@ -210,6 +210,9 @@ EOTEXT
           // This isn't a standard handler installed by an application, but
           // is a reasonable name for a user-installed handler.
           'editor' => true,
+
+          // This handler is for Visual Studio Code.
+          'vscode' => true,
         ))
         ->setSummary(pht('Whitelists editor protocols for "Open in Editor".'))
         ->setDescription(