Migrate scripts to ESM and ts-check

scripts/utils/assert.mjs

@@ -0,0 +1,22 @@
+// Copyright 2025 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+// @ts-check
+
+/**
+ * @param {boolean} condition
+ * @param {string} message
+ * @returns {asserts condition}
+ */
+export function assert(condition, message) {
+  if (!condition) {
+    throw new TypeError(message);
+  }
+}
+
+/**
+ * @param {never} value
+ * @returns {never}
+ */
+export function unreachable(value) {
+  throw new TypeError(`Expected case to be unreachable, found ${value}`);
+}