Add a Yarn lock file

[vanderwees]

Let's add a Yarn (npm alternative) lock file, so we're sure to work with the same package versions.

[derhuerst]

Note that people who may not use Yarn for package management won't get the same package versions.

[jfilter]

I am fine with having both, npm and yarn lock file. We can also switch to Yarn all togehter.

[derhuerst]

People will need to remember to update both the npm and yarn lock files. That was all I wanted to say.

[jfilter]

I get your point. Ideally, we should stick to one. Let's not overengineer it for now.