fix: add package exports and build for git installs

[mridang]

Description

Added main, types, and exports pointing to dist/index so consumers can import from the package root. Added a prepare script so dist is built during git installs.

Related Issue

None

Motivation and Context

GitHub installs were missing dist, causing ERR_MODULE_NOT_FOUND when importing @zitadel/zitadel-node/dist/index.js. Root-level imports also failed because no entrypoint was declared.

How Has This Been Tested?

Ran npm run build and then inn a fresh temp project using Node 20, we ran.

npm init -y
npm_config_cache="$PWD/.npm-cache" 
npm install ../

Once linked, we validated that the issue was resolved since we are now able to use proper ESM imports.

import Zitadel, { ApiException } from '@zitadel/zitadel-node';

Documentation:

The Readme is up to date.

Checklist:

• I have updated the documentation accordingly.
• I have assigned the correct milestone or created one if non-existent.
• I have correctly labeled this pull request.
• I have linked the corresponding issue in this description.
• I have requested a review from at least 2 reviewers
• I have checked the base branch of this pull request
• I have checked my code for any possible security vulnerabilities

[mridang]

@matiasracedo I'll get this out soon. FYI.