- Convert source code from CommonJS to TypeScript (src/index.ts)
- Replace Babel with Vite for building ESM and CJS bundles
- Replace Flow with TypeScript for type checking
- Replace Jest with Vitest for testing
- Migrate from yarn to pnpm as package manager
- Add ESLint flat config with TypeScript support
- Remove legacy config files (Babel, Flow, Travis CI, Renovate)
- Generate proper TypeScript declarations with vite-plugin-dts

- Add Makefile with targets: install, build, test, lint, typecheck, clean, ci
- Add GitHub Actions workflow that runs CI on Node 18, 20, and 22
- Workflow triggers on push/PR to master/main branches

- Replace Travis CI badge with GitHub Actions badge
- Update install examples to show pnpm first
- Change usage example to ESM import syntax
- Add TypeScript usage section with Comparator type example
- Update development section with pnpm and Make commands
- Update prerequisites to Node.js v18+ and pnpm

Expand test coverage from 16 to 79 test cases organized into categories:
- Primitive values: strings, numbers, booleans, symbols, bigints
- Special values: NaN, +0/-0, Infinity, null, undefined
- Objects: empty, shallow equal, key order, inherited/non-enumerable props
- Arrays: empty, same elements, different lengths, sparse arrays
- Mixed types: array vs object, Date, RegExp, Map, Set, functions
- Customizer function: arguments, this binding, return value handling
- Edge cases: numeric keys, undefined values, many keys, circular refs,
 typed arrays, ArrayBuffer, Error, Promise, WeakMap/WeakSet
Tests document actual shallowequal behavior including that objects with
no enumerable own properties (Date, RegExp, Map, Set) compare as equal.