Change standalone Action file from CommonJS to ESModule

[ericcornelissen]

Since #592 most of this project's JavaScript uses ESModules syntax. However, the standalone JavaScript file that is run when the Action is used (lib/index.cjs) still uses CommonJS syntax.

Ideally, the build process is updated to bundle the Action into an ESModule-based standalone file. Provided that is works (this would be verified by the end-to-end tests) and is otherwise sensible.