Add rushx support too

Co-authored-by: Copilot <copilot@github.com>

packages/safe-chain/src/packagemanager/rushx/createRushxPackageManager.js

@@ -0,0 +1,18 @@
+import { runRushCommand } from "../rush/runRushCommand.js";
+
+/**
+ * @returns {import("../currentPackageManager.js").PackageManager}
+ */
+export function createRushxPackageManager() {
+  return {
+    /**
+     * @param {string[]} args
+     */
+    runCommand: (args) => {
+      return runRushCommand("rushx", args);
+    },
+    // For rushx, rely solely on MITM.
+    isSupportedCommand: () => false,
+    getDependencyUpdatesForCommand: () => [],
+  };
+}