* Extract @vercel/prepare-flags-definitions package
Extract the core flag definitions preparation logic from the Vercel CLI into a standalone, reusable package. This includes:
- prepareFlagsDefinitions() function that collects SDK keys from env, fetches definitions, and writes to node_modules
- hashSdkKey() helper for SHA-256 hashing
- generateDefinitionsModule() for JS module generation with deduplication and lazy parsing
Follows repo conventions: pnpm workspaces, tsup, vitest, ES modules with CJS fallback.
* Add JSDoc comments from original CLI source
* Add inline comments from original CLI source
* Add optional output parameter with debug logging
* Add output.time for datafile fetching
* Add changeset for @vercel/prepare-flags-definitions
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