feat: add Subscription Items to Node SDK (#160)

[Abhishek-singh88]

Description

Resolves #160

This PR adds the SubscriptionItems resource to the Node SDK to expose the new public endpoints introduced in #159.

• Created the SubscriptionItems resource class, matching Stripe's SDK surface by providing create, retrieve, update, del, and list methods.
• Reused existing Zod schemas (CreateSubscriptionItemInput, etc.) and types from @zoneless/shared-schemas and @zoneless/shared-types.
• Wired the new resource into the main Zoneless client export (zoneless.subscriptionItems).

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Documentation

How Was This Tested?

• Unit tests

• Integration tests

• Manual testing

• Added comprehensive unit tests in SubscriptionItems.spec.ts to verify HTTP paths, methods, and payload mapping against the HttpClient mock for all 5 methods.

• Verified successful compilation and test execution via npx nx build node-sdk and npx nx test node-sdk.

Checklist

• Self-reviewed my code
• No new warnings
• Existing tests still pass
• Breaking changes are documented above

PR Checklist

• Tests pass (npx nx run-many --target=test --all)
• Linting passes (npx nx run-many --target=lint --all)
• Formatting passes (npm run format:check)
• No merge conflicts with main

[Benzilla]

Looks good, thank you. Merging in now!