refactor: reuse createTextEvent() in createAckEvent() (#109)

### What are you trying to accomplish?

A refactoring opportunity: after reading `createAckEvent()` and
`createTextEvent`'s definitions, I noticed that the only difference was
that the former passes in an empty string for its content.

### What approach did you choose and why?

I chose to update the body to use `createTextEvent()` directly, passing
an empty string for its `message` argument. Ran tests locally and in CI
to verify this works 👍

### What should reviewers focus on?

Nothing I haven't mentioned 😉

lib/response.js

@@ -2,15 +2,7 @@
 
 /** @type {import('..').CreateAckEventInterface} */
 export function createAckEvent() {
-  const data = {
-    choices: [
-      {
-        index: 0,
-        delta: { content: ``, role: "assistant" },
-      },
-    ],
-  };
-  return `data: ${JSON.stringify(data)}\n\n`;
+  return createTextEvent("");
 }
 
 /** @type {import('..').CreateTextEventInterface} */