fix: 🚑 fix default export

Due to this bug, `llmstxt` could not be imported via default import:

```ts
import llmstxt from 'vitepress-plugin-llms'
```

And it had to be imported through `{ }`:

```ts
import { llmstxt } from 'vitepress-plugin-llms'
```

Now it fixed.

🦁 Spotted in #62 (comment)

Reported-by: Lakr <git@qaq.wiki>

Author: okineadev

src/index.ts

@@ -1,2 +1,2 @@
 export { copyOrDownloadAsMarkdownButtons } from './markdown/markdown-it-plugins'
-export { llmstxt } from './plugin/main'
+export { llmstxt as default } from './plugin/main'