Add `yarn pnp` support

[KevinEdry]

Currently if you are using yarn pnp with this package, you will (understandably) get this error message:

Attempting to initialize using the npm package cz-conventional-changelog
Error: Could not locate node_modules in your project's root directory. Did you forget to npm init or npm install?
TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined

This is after installing commitizen locally via:

yarn add -D commitizen

and then running to following init command:

yarn dlx commitizen init cz-conventional-changelog --yarn --save-dev --save-exact

This is because yarn pnp does not use node_modules folder to install it's packages.