DevKit: restore the old vendor validation logic

from IDEA-CR-52434:
This method is called not only for bundled plugins, but also for custom plugins. We cannot be sure that plugin.xml for all such plugins are updated? E.g. there is JsTestDriver plugin (https://github.com/JetBrains/intellij-obsolete-plugins/blob/master/JsTestDriver/plugin/src/main/resources/META-INF/plugin.xml) that specifies "Google, JetBrains".

GitOrigin-RevId: d6764300b9c9d5842cda56e0c2df862415e54110

platform/platform-impl/src/com/intellij/ide/plugins/PluginManagerMain.java

@@ -60,7 +60,13 @@ public abstract class PluginManagerMain {
   }
 
   public static boolean isDevelopedByJetBrains(@Nullable String vendorString) {
-    return JETBRAINS_VENDOR.equals(vendorString);
+    if (vendorString == null) return false;
+    for (String vendor : StringUtil.split(vendorString, ",")) {
+      if (vendor.trim().equals(JETBRAINS_VENDOR)) {
+        return true;
+      }
+    }
+    return false;
   }
 
   private static String getTextPrefix() {