Moves stringToArrayBuffer into util folder

ts/util/stringToArrayBuffer.ts

@@ -0,0 +1,14 @@
+// Copyright 2018-2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+export function stringToArrayBuffer(string: string): ArrayBuffer {
+  if (typeof string !== 'string') {
+    throw new TypeError("'string' must be a string");
+  }
+
+  const array = new Uint8Array(string.length);
+  for (let i = 0; i < string.length; i += 1) {
+    array[i] = string.charCodeAt(i);
+  }
+  return array.buffer;
+}