Add ElectronShellOpenExternalSink class

Add ElectronShellOpenExternalSink class to detect untrusted input
interpreted by `openExternal` function call in `electron` module.

Based on the #14 Electron Security checklist:
https://www.electronjs.org/docs/tutorial/security#14-do-not-use-openexternal-with-untrusted-content

javascript/ql/src/semmle/javascript/security/dataflow/CodeInjectionCustomizations.qll

@@ -138,4 +138,15 @@ module CodeInjection {
         API::moduleImport("module").getInstance().getMember("_compile").getACall().getArgument(0)
     }
   }
+
+  /**
+   * Improper use of openExternal can be leveraged to compromise the user's host.
+   * When openExternal is used with untrusted content, it can be leveraged to execute arbitrary commands.
+   */
+  class ElectronShellOpenExternalSink extends Sink {
+    ElectronShellOpenExternalSink() {
+      this =
+        DataFlow::moduleMember("electron", "shell").getAMemberCall("openExternal").getArgument(0)
+    }
+  }
 }