fix: change postinstall for prepare script

[0xnigir1]

Motivation and Context

When installing this package directly from GitHub, it needs to be built during installation. Right now, this uses a postinstall script, but devDependencies (e.g., tsc from typsecript) aren’t installed, causing the build to fail. The correct approach is to use a prepare script, as it ensures devDependencies are installed too and the script runs before packaging and installation.

Taken from npm Docs:

NOTE: If a package being installed through git contains a prepare script, its dependencies and devDependencies will be installed, and the prepare script will be run, before the package is packaged and installed.

References:
scripts | npm Docs

Changes

• change postinstall for prepare script in package.json