chore: add storybooks

[ijsnow]

This PR adds storybooks to this project. This project is a react component library. This means that it should be able to run in many places out of the box. For developing libraries like this, tools like Storybooks are great. It's a fast development loop and forces you to write your code in modular and reusable ways since its an isolated environment. This means that you'll also move faster in the future since the code will be inherently more maintainable. In addition to this, Storybooks serves as documentation and can be used to integrate with unit tests and much more.

I'm not actively working on this project so I'm just going to leave this here for @chrismwendt and @sqs to decide if they want it. This project is the perfect use case for this type of tool and I encourage you to give it a try but it's up to you the owners and active maintainers of the project.

[chrismwendt]

I tried this out locally and I like how fast the feedback loop is. I'm not sure how often I'd use this, but at least I know how to add it to a project now 🙇

[ijsnow]

It would also help in situations where we want others to just look at styles like in #48.

[chrismwendt]

Here's how to move this PR to https://github.com/sourcegraph/sourcegraph/tree/master/packages/sourcegraph-extension-api

cd browser-extensions
git format-patch master --stdout > /tmp/patch

cd ../sourcegraph
cat /tmp/patch | git am -3 --directory=packages/sourcegraph-extension-api/
# and fixup merge conflicts