feat: add support for stdin JSON input

[aduh95]

The goal is to remove the need to clone the repo in https://github.com/nodejs/node/blob/main/.github/workflows/notify-on-push.yml, and instead rely on push.commits like so:

- name: Validate commits
  run: echo "$COMMITS" | npx -q core-validate-commit -
  env:
    COMMITS: ${{ toJSON(github.event.commits) }}