feat: switch to rollup for dual ESM/CJS output

- Replace tsc with rollup to produce both .js (ESM) and .cjs (CJS) bundles
- Add "type": "module" and "exports" field to package.json
- Set "main" to lib/index.cjs for CJS backward compatibility
- Add "module" field for bundlers
- Update tsconfig: target ES2015, module ES2015, moduleResolution bundler
- Remove importHelpers (no runtime tslib dependency)
- Move tslib to devDependencies (required by @rollup/plugin-typescript)
- Remove tsconfig.test.json (unified tsconfig)
- Update devDependencies to latest versions
- Remove mocha, ts-mocha, @types/chai, @types/mocha from dependencies

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people