Modernize to TypeScript and current Node LTS

[philihp]

Summary

Modernizes this tiny library along a few axes — same public API, no behavior change:

• TypeScript source. src/index.js → src/index.ts; tests too. tsup now emits .d.ts and a types condition is added to package.json#exports, so consumers get types. TypeScript was already a devDep but wasn't doing anything.
• Stop reallocating on every call. The ArrayBuffer/Float32Array/Int32Array are hoisted to module scope — they were being reallocated on every toNaN/fromNaN call. Single-threaded JS makes this safe (these functions are synchronous and atomic).
• Name the default export (nanbox) instead of leaving it anonymous.
• Tests use Node's built-in TS stripping (node --experimental-strip-types --test). No tsx/ts-node/extra test runner.
• Node baseline. engines.node → >=22.6 (required for type stripping). CI drops EOL Node 18 and near-EOL Node 20; runs on 22.x and 24.x. .node-version bumped.
• CI also runs tsc --noEmit so type errors fail the build.
• Coveralls action bumped from @master to @v2 (pinned, with current input names).
• Cleaned up stale .npmignore entries (.babelrc, nyc.config.js, removed README.md which npm needs).

Test plan

• npm run build — emits dist/index.{js,cjs,d.ts,d.cts}
• npx tsc --noEmit — clean
• npm test — all 5 tests pass under --experimental-strip-types
• Smoke-test the built artifacts: ESM and CJS import/require both round-trip a codepoint through nanbox and toNaN/fromNaN
• npx eslint — clean
• CI matrix (22.x, 24.x) passes on GitHub Actions

https://claude.ai/code/session_01RyCEnf6V2vWGWRsRZQrjFP

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[philihp]

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