Add yarn.lock

Yarn (https://yarnpkg.com) is like npm, but you don’t have do delete
`node_modules` each time you have a bug. It is also faster.

`yarn.lock` store exactly which versions of each dependency were
installed.

From https://yarnpkg.com/en/docs/yarn-lock:

All yarn.lock files should be checked into source control (e.g. git or
mercurial). This allows Yarn to install the same exact dependency tree
across all machines, whether it be your coworker’s laptop or a CI
server.

Framework and library authors should also check yarn.lock into source
control. Don’t worry about publishing the yarn.lock file as it won’t
have any effect on users of the library.