feat: Sharding

[Mike-Dax]

We have a growing list of stories to screenshot and we've had trouble scaling vertically on the one computer. Splitting the work up via multiple storybook servers has become hard to handle at scale. Instead we've added a feature to storycap to allow for 'sharding' on the screenshot end.

By adding --shard <shardNumber>/<totalShards> and running storycap on multiple computers, we can horizontally scale the screenshot process.

Stories are sorted by their IDs then distributed across shards in a round-robin style. We have some stories that rely on WebGL which are much slower than other simple components, since they're named similarly, this method reasonably distributes them across our workers.

For example, to run across two computers:

storycap --shard 1/2
storycap --shard 2/2

Merging the resultant screenshot artifacts is left to user, our CI solution makes that pretty easy and it seems out of the scope of storycap.

I hope this is helpful!