Convert GitHub Action to ESM

[matejchalk]

The GitHub Actions currently runs as CommonJS, which means it can't import ESM-only packages. Therefore it can't import any packages from code-pushup/cli repo, which is a blocker for code-pushup/cli#829.

Acceptance criteria

• convert from CJS to ESM
• ensure tests still work (Jest has experimental support, alternative is to switch to Vitest)