feat: make the API async

[gregberge]

BREAKING CHANGE: API is now returning a promise

[Copilot]

Pull request overview

This PR converts the fingerprintDiff API from synchronous to asynchronous by implementing NAPI's AsyncTask pattern, enabling non-blocking execution of the image fingerprinting operations.

Changes:

• Refactored the Rust implementation to use AsyncTask with a new FingerprintDiffTask struct that implements the Task trait
• Updated TypeScript type definitions to return Promise<unknown> instead of string
• Updated all test cases to use async/await syntax

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/lib.rs	Introduced FingerprintDiffTask struct implementing Task trait; moved computation logic to compute() method for async execution; updated fingerprint_diff to return AsyncTask
index.d.ts	Changed return type from string to Promise<unknown>
test/index.spec.ts	Updated test functions to be async and added await keywords for all fingerprintDiff calls
README.md	Updated usage example to include await keyword and documented that the function returns a Promise<string>

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