Add yarn lock file

[nietonfir]

Yarn is an npm compatible drop-in replacement optimized (not only) for speed, determinism, offline-mode, … ;-) For more information about yarn see https://yarnpkg.com/

[DavidBadura]

The yarn.lock file will be ignored if you installed this package as dependency. So it doesn't matter what is in the lock file. ;-)

[nietonfir]

That is correct. The intention for adding the lock file would be mostly for plugin developers (since we're only dealing with devDependencies), to ensure a consistent stack across them. And travis obviously, since it's supported now.