Add TypeScript declarations for admin-api

[Josejj10]

Summary

Add first-party TypeScript declarations for @tryghost/admin-api and type tests.

• This provides built-in types to consumers for a better experience: editor IntelliSense, compile-time safety.
• No runtime changes; library behavior remains the same.
• No breaking changes.
• Solves TypeScript support for the admin api client #446

Checklist

• There’s a clear use-case for this code change
• Commit message has a short title & references relevant issues
• The build will pass (run yarn test and yarn lint)

Changes

• Add packages/admin-api/index.d.ts covering API resources: posts, pages, tags, members, users, newsletters, webhooks, uploads, config, site, themes.
• Expose types via types: "index.d.ts" and include in files in packages/admin-api/package.json.
• Add type tests under packages/admin-api/test/types:
  • tsconfig.json (self-contained)
  • types.test.ts using inline generic assertions so tsc enforces types without extra libs.
• Add test:types script and wire it in CI if desired (currently separate).

Usage example

import GhostAdminAPI = require('@tryghost/admin-api');

const api = new GhostAdminAPI({
  url: <url>,
  key: <key>,
  version: 'v5.0',
});


const posts = await api.posts.browse({ limit:10, page:1 })

Screenshots