Update partner README, add TypeScript typings, and migrate webhook flows to v3 upsert

[vivek-gofynd]

Motivation

• Make onboarding and integration clearer for external partners by focusing docs and examples on partner use-cases.
• Provide TypeScript typings so consumers can get IDE/type support when importing the Express helper and storage adapters.
• Remove legacy webhook subscriber APIs and unnecessary per-provider fallback logic to standardize on the v3 upsert flow.
• Add explicit saleschannel scoping support (subscribed_saleschannel_ids) and a single upsert path to simplify sync semantics.

Description

• Rewrote the README into a partner-focused guide with installation, quick-start, webhook configuration, saleschannel scoping, and examples for routes and background usage (README.md).
• Added TypeScript declaration files and package wiring: index.d.ts, express/index.d.ts, and express/storage/index.d.ts, and set "types": "index.d.ts" in package.json to expose typings.
• Simplified express/webhook.js webhook syncing: removed the legacy per-provider v1/v2 fallback registration/update/get flows and saleschannel enable/disable helpers and now always upserts via the v3.0 put API from syncSubscriberConfigForAllProviders; added support for subscribed_saleschannel_ids and enableWebhooks in payload generation (createRegisterPayloadData).
• Removed manual subscribe/unsubscribe example endpoints from examples/test.js and updated CHANGELOG.md with an Unreleased entry describing the changes.

Testing

• No automated tests were executed as part of this change (test suite not run).
• All changes were committed; manual review of modified files was performed during the rollout.

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Codex Task