feat: options.id can be set to Client ID

[gr2m]

This pull request updates the handling and documentation of the id argument to accept both a number (App ID) or a string (Client ID), enhancing flexibility in accordance with GitHub's recommendations.

• Updates type definitions: Modifies index.d.ts and internals.d.ts to allow the id parameter and appId result to be either a number or a string.
• Revises documentation: Adjusts README.md to clarify that options.id can be set to either the App ID or Client ID, and recommends using the Client ID for github.com and GHES 3.14+. The resulting appId can also be a string accordingly.
• Expands test coverage: Adds a new test case in index.test-d.ts and test/node.test.js to verify functionality when id is set to a string.

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[gr2m]

To avoid friction for downstream users, I think would make sense to implement the appId type of the result to be inferred by the id type passed to the function

Update: done ✅

[github-actions]

🎉 This PR is included in version 2.2.0-beta.1 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

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