fix: change simple-git-hooks from postinstall to prepare

[ryohidaka]

Summary

Change postinstall to prepare on package.json

Why

When a library created with Bunup is installed via npm install in an environment where Bun is not installed, an error occurs.

How

The post-install script runs both on the end-user's side, so change it to prepare to ensure it only runs in the developer's environment.

[arshad-yaseen]

Yeah, thank you so much! It's was a critical issue, we already fixed this in bunup repo yesterday: bunup/bunup@590e0cd