Remove jsonwebtoken and base64url in favor of native crypto (#318)

[shubhransh-gupta]

Summary

Replaces jsonwebtoken and base64url dependencies with Node.js built-in crypto (sign, verify, and Buffer.from(..., 'base64url')).

Fixes #318

Motivation & Problem

jsonwebtoken has unmaintained transitive dependencies (jws, jwa, ecdsa-sig-formatter, safe-buffer) that cause compatibility and packaging errors when used with modern ESM bundlers (such as esbuild, Vite, Rollup, webpack) and alternative runtimes (such as Bun).

By using standard Node.js built-in crypto primitives:

1. Zero JWT runtime dependencies: Completely drops jsonwebtoken, @types/jsonwebtoken, base64url, and their 20+ transitive dependencies.
2. ESM / Bundler Compatibility: Eliminates bundler errors caused by legacy CommonJS sub-dependencies like safe-buffer.
3. No breaking changes: All public methods and signatures retain identical behavior and synchronous/asynchronous contracts.

What Changed

• index.ts: Implemented createBearerToken() using Node's sign('SHA256', ..., { key, dsaEncoding: 'ieee-p1363' }).
• jws_signature_creator.ts: Implemented internalCreateSignature() using Node's built-in crypto.sign and Buffer.from(..., 'base64url').
• jws_verification.ts: Decodes JWT headers and payloads using native base64url buffer conversions, and verifies ES256 signatures with Node's verify('SHA256', ..., { key: publicKey, dsaEncoding: 'ieee-p1363' }, ...).
• package.json & yarn.lock: Removed jsonwebtoken, @types/jsonwebtoken, and base64url.
• NOTICE.txt: Updated third-party notices accordingly.
• Tests (api_client.test.ts, jws_signature_creator.test.ts, util.ts): Updated test utilities and mocks to use native crypto / base64url decoding.

Test Plan

• npm run build (tsc -p .) builds cleanly with zero errors.
• npm test passes all 22 test suites and 344/344 unit tests.
• npm run build-docs generates TypeDoc documentation with 0 errors.

[shubhransh-gupta]

Hi @alexanderjordanbaker — Friendly reminder on this PR when you have time for a review. It resolves #318 by migrating from jsonwebtoken and base64url to Node.js native crypto. Full TypeScript build and test suite pass with 100% test coverage. Thank you!