Publish as cz instead of commitizen on NPM

It would be awesome if we could keep consistency between the CLI name `cz` and the NPM package name, `commitizen`.

This would allow people to:

1. Use `npx cz` everywhere, even when not installed. Refs https://github.com/commitizen/cz-cli/issues/762#issuecomment-692109333.
2. Set up their repositories easily with `npm i -D cz`.
3. Get rid of the confusion caused by `npx git-cz`, which uses @streamich's [git-cz](https://github.com/streamich/git-cz) instead of commitizen's official, by advertising `npx cz` more instead of `npx git-cz`.

For this to happen we would need:

1. Maintainers approval
2. @OmgImAlexis approval, because she owns the[ `cz` namespace on NPM](https://www.npmjs.com/package/cz). What do you think, @OmgImAlexis?