Add no-misused/floating-promises lint rule

ts/util/drop.ts

@@ -0,0 +1,8 @@
+// Copyright 2022 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+// Use this function when asynchronous action has to run without blocking its
+// parent.
+export function drop(promise: Promise<unknown> | undefined): void {
+  void promise;
+}