Migrate from Github Actions v2 to v3

[archive]

Migrated the codebase from CommonJS to ES Modules to support @actions/core v3, which dropped CommonJS support.

• Source files: converted all require()/module.exports to import/export across 13 files.
• Jest: renamed jest.config.js -> jest.config.cjs (keeps CJS syntax while the rest of the project is ESM), updated test script to run with --experimental-vm-modules, and replaced jest.mock() with
  jest.unstable_mockModule() (the ESM-compatible mocking API).
• Test files that use the jest object now explicitly import it from @jest/globals.
• ESLint: updated sourceType from "commonjs" to "module".
• package.json: added "type": "module" and bumped @actions/core to ^3.0.0.

Bonus: Removed dependency "@actions/github" since it was not needed anymore.