Convert to ESM

[ncalteen]

This PR converts the JavaScript action template to ESM. Given that some of the core dependencies used in GitHub Actions (namely octokit/rest.js) are being converted to modules, it is necessary to make this change in new actions going forward.

The core changes are as follows:

• Switch from @vercel/ncc to rollup for packaging the transpiled action code
• Configure Jest to handle ESM module loading and proper mocking of dependencies
• Upgraded ESLint to 9.x (requires update from YAML to flat config format

Some additional changes that are included for convenience/ease of use:

• Added links/references to specific linting configuration files
• Added information on updating the CODEOWNERS file
• Added test fixtures to stub @actions/core during unit tests
• Added information on enforcing unit test coverage (optional)