chore: configure chromatic snapshot tests (#896)

Resolves: #444

Summary:

This commit installs and configures a GH action for chromatic. Chromatic
is used for snapshot testing build-over-build.

Details:

* chore: install chromatic

* chore: add chromatic package.json script

Suggested by the docs for convenience so that we can run chromatic like:

```console
yarn run chromatic ...
```

* chore: gitignore storybook builds

* ci: configure chromatic

This action configures chromatic to run in CI on pushes to all branches.
By running this in CI, we get the following:

- snapshot (build-over-build)
- checks in our CI

The snapshots and build-over-build behavior are per branch; this way we
can work on a feature branch without worrying about changes being made
to mainline independently.

* chore: remove manual storybook build from CI

This is now the responsibility of Chromatic

.gitignore

@@ -25,6 +25,7 @@ site/test-results/
 site/yarn-error.log
 coverage/
 site/**/*.typegen.ts
+site/build-storybook.log
 
 # Build
 dist/