Use cosmiconfig so users can use whatever config file extension they want

With `cosmiconfig` we could automatically load the configuration file from whatever supported file, such as:

> - a package.json property
> - a JSON or YAML, extensionless "rc file"
> - an "rc file" with the extensions .json, .yaml, .yml, .js, or .cjs
> - a .config.js or .config.cjs CommonJS module

I'm personally looking for a `.czrc.json`, so my VSCode can automatically detect it as JSON.

Nonetheless, this would reduce a lot of code from this repo.

https://github.com/davidtheclark/cosmiconfig

Related: https://github.com/commitizen/cz-cli/issues/460