Improve support for Yarn

[mischah]

Hej,

this is a follow up to #964.
I just checked changes of 4eccb2a and I see a problem with how Yarn install is implemented. Or let’s say its missing one thing:

Dependencies should be installed with yarn add lodash or yarn add --dev lodash.
See: https://yarnpkg.com/en/docs/cli/install

Installing dependencies with yarn install lodash --save or yarn install lodash --save works for now but is deprecated since the first public release of Yarn:

So it’s just a matter of time until it breaks.

We should extend the runinstall method to call yarn add instead of yarn install for installing dependencies.

[SBoudrias]

Yup, totally should

[mischah]

Nice 💖