Migrate to ES modules (ESM) (#166)

* Upgrade @actions/core to v3 and @actions/tool-cache to v4

These packages are now ESM-only, which is incompatible with @vercel/ncc's
CJS bundling. Switch the bundler from ncc to esbuild, set package type to
"module", update tsconfig to use node16 module resolution, and rename
jest.config.js to jest.config.cjs for CJS compatibility.

https://claude.ai/code/session_01QwnXfWH2ELHXvNgijYTeM1

* Fix CI: use bundler module resolution instead of ESM package type

The "type": "module" in package.json caused the CJS esbuild bundle to be
parsed as ESM at runtime, breaking require() calls for Node.js built-ins.
Instead, use moduleResolution: "bundler" in tsconfig which resolves ESM
exports fields correctly while keeping the project as CJS. Revert
jest.config.cjs back to jest.config.js.

https://claude.ai/code/session_01QwnXfWH2ELHXvNgijYTeM1

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: kyleconroy