Use `yarn` if available

I started using yarn since the npm fiasco.

It’s a pain to remove dependencies installed after npm install & then manually doing yarn.

I think it would be much nice if yarn is used if it’s installed or just give me an option prisma2 init example --yarn so that I can directly install using yarn.

[sidharthachatterjee]

Hello from Gatsby! Here is how we handle this:

• We ask the user to pick a preferred package manager on first run and persist that in ~/.config
• We include lock files for both in official starters and delete the appropriate one on gatsby new ...

This is at https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-cli/src/init-starter.js#L30

[divyenduz]

This is done in the latest preview. npm install -g prisma2