chore: add vercel example

[yusinto]

This supersedes earlier prs to add a vercel example because:

1. Package linking via yarn workspace works now.
2. This uses the new EdgeFeatureStore class.

[shortcut-integration]

This pull request has been linked to Shortcut Story #199365: Add vercel example.

[yusinto]

@monsagri are you able to compare this example app with your private one to debug the package.json issue?

[louis-launchdarkly]

I don't have vercel to run this, but the code looks fine and the build is green. You may want to wait for another Approval if you want someone who have more JavaScript experience reviewing this.

[yusinto]

Ta @louis-launchdarkly this is just an example app and I think it's ok to merge now for the benefit of our customers. I tested this thoroughly so it should be ok. We can fix issues as they arise. Thank you!