feat(setup-node): add cache input for npm caching

[GoogilyBoogily]

Summary

Adds a cache input to the setup-node composite action, threaded through to actions/setup-node@v6's built-in caching.

• Default is "npm", so all existing consumers get caching with zero changes on their end
• Set to empty string to disable caching
• Cache key generation is handled automatically by actions/setup-node@v6 (hashes package-lock.json)

Test plan

• Trigger CI in any consumer repo — second run shows "Cache hit" in setup-node logs
• Confirm no consumers need to be updated

[Copilot]

Pull request overview

Adds configurable dependency caching to the repo’s actions/setup-node composite action by exposing and wiring through the cache input to actions/setup-node@v6, allowing consumers to enable caching by default or disable it explicitly.

Changes:

• Introduces a new cache input (default: "npm") on the composite action.
• Passes the cache input through to the underlying actions/setup-node@v6 step.

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