Update prismTheme.js export to use CommonJS syntax (#9)

Tried running `yarn build` but it didn't work. I asked Claude for help and it told me I had to update this line and then it worked. I tried looking at how we did it with the `docs/eval-docs` part of `lmai` repo (https://github.com/lastmile-ai/lmai/blob/main/docs/eval-docs/tsconfig.json), but couldn't get it to work with those changes. 

This gets me unblocked so doing it for now and putting in really easy line so that it's easy to review

## Test Plan
```bash
yarn build
```

website/core/prismTheme.js

@@ -123,4 +123,4 @@ const theme = {
     ],
   };
 
-  export default theme;
+module.exports = theme;