reduce browser bundle size

reduces browser bundle size by ~612k or ~92%.

src/util/node-support.ts

@@ -0,0 +1,7 @@
+import { createHmac } from 'crypto';
+
+export async function signMessage(message: string, secret: string): Promise<string> {
+  return createHmac('sha256', secret)
+    .update(message)
+    .digest('hex');
+};